diff --git a/lib/openapi_first.rb b/lib/openapi_first.rb
index f90ec527..cb41d0a3 100644
--- a/lib/openapi_first.rb
+++ b/lib/openapi_first.rb
@@ -52,9 +52,13 @@ def self.find_error_response(name)
     end
   end
 
-  # Load and dereference an OpenAPI spec file
+  # Load and dereference an OpenAPI spec file or return the Definition if it's already loaded
+  # @param filepath_or_definition [String, Definition] The path to the file or a Definition object
   # @return [Definition]
-  def self.load(filepath, only: nil, &)
+  def self.load(filepath_or_definition, only: nil, &)
+    return filepath_or_definition if filepath_or_definition.is_a?(Definition)
+
+    filepath = filepath_or_definition
     raise FileNotFoundError, "File not found: #{filepath}" unless File.exist?(filepath)
 
     contents = FileLoader.load(filepath)
diff --git a/lib/openapi_first/definition.rb b/lib/openapi_first/definition.rb
index 6d4863ce..cf8bd530 100644
--- a/lib/openapi_first/definition.rb
+++ b/lib/openapi_first/definition.rb
@@ -40,6 +40,24 @@ def initialize(contents, filepath = nil)
     # @return [Enumerable[Router::Route]]
     def_delegators :@router, :routes
 
+    # Returns a unique identifier for this API definition
+    # @return [String] A unique key for this API definition
+    def key
+      return filepath if filepath
+
+      info = self['info'] || {}
+      title = info['title']
+      version = info['version']
+
+      if title.nil? || version.nil?
+        raise ArgumentError,
+              "Cannot generate key for the OpenAPI document because 'info.title' or 'info.version' is missing. " \
+              'Please add these fields to your OpenAPI document.'
+      end
+
+      "#{title} @ #{version}"
+    end
+
     # Validates the request against the API description.
     # @param [Rack::Request] request The Rack request object.
     # @param [Boolean] raise_error Whether to raise an error if validation fails.
diff --git a/lib/openapi_first/test.rb b/lib/openapi_first/test.rb
index 9be47f64..0515e2b2 100644
--- a/lib/openapi_first/test.rb
+++ b/lib/openapi_first/test.rb
@@ -103,18 +103,23 @@ class AlreadyRegisteredError < StandardError; end
     class << self
       attr_reader :definitions
 
-      def register(path, as: :default)
-        if definitions.key?(:default)
+      # Register an OpenAPI definition for testing
+      # @param path_or_definition [String, Definition] Path to the OpenAPI file or a Definition object
+      # @param as [Symbol] Name to register the API definition as
+      def register(path_or_definition, as: :default)
+        if definitions.key?(as) && as == :default
           raise(
             AlreadyRegisteredError,
             "#{definitions[as].filepath.inspect} is already registered " \
-            "as ':default' so you cannot register #{path.inspect} without " \
+            "as ':default' so you cannot register #{path_or_definition.inspect} without " \
             'giving it a custom name. Please call register with a custom key like: ' \
-            "OpenapiFirst::Test.register(#{path.inspect}, as: :my_other_api)"
+            "OpenapiFirst::Test.register(#{path_or_definition.inspect}, as: :my_other_api)"
           )
         end
 
-        definitions[as] = OpenapiFirst.load(path)
+        definition = OpenapiFirst.load(path_or_definition)
+        definitions[as] = definition
+        definition
       end
 
       def [](api)
diff --git a/lib/openapi_first/test/coverage.rb b/lib/openapi_first/test/coverage.rb
index 935b7a6d..2c35d631 100644
--- a/lib/openapi_first/test/coverage.rb
+++ b/lib/openapi_first/test/coverage.rb
@@ -36,7 +36,7 @@ def install
         def start(skip_response: nil)
           @current_run = Test.definitions.values.to_h do |oad|
             plan = Plan.for(oad, skip_response:)
-            [oad.filepath, plan]
+            [oad.key, plan]
           end
         end
 
@@ -53,11 +53,11 @@ def reset
         end
 
         def track_request(request, oad)
-          current_run[oad.filepath].track_request(request)
+          current_run[oad.key].track_request(request)
         end
 
         def track_response(response, _request, oad)
-          current_run[oad.filepath].track_response(response)
+          current_run[oad.key].track_response(response)
         end
 
         def result
diff --git a/lib/openapi_first/test/coverage/plan.rb b/lib/openapi_first/test/coverage/plan.rb
index 99a0faef..da0ac2a5 100644
--- a/lib/openapi_first/test/coverage/plan.rb
+++ b/lib/openapi_first/test/coverage/plan.rb
@@ -13,7 +13,7 @@ class Plan
         class UnknownRequestError < StandardError; end
 
         def self.for(oad, skip_response: nil)
-          plan = new(filepath: oad.filepath)
+          plan = new(definition_key: oad.key, filepath: oad.filepath)
           oad.routes.each do |route|
             responses = skip_response ? route.responses.reject(&skip_response) : route.responses
             plan.add_route request_method: route.request_method,
@@ -24,13 +24,14 @@ def self.for(oad, skip_response: nil)
           plan
         end
 
-        def initialize(filepath:)
+        def initialize(definition_key:, filepath: nil)
           @routes = []
           @index = {}
+          @api_identifier = filepath || definition_key
           @filepath = filepath
         end
 
-        attr_reader :filepath, :routes
+        attr_reader :api_identifier, :filepath, :routes
         private attr_reader :index
 
         def track_request(validated_request)
diff --git a/lib/openapi_first/test/coverage/terminal_formatter.rb b/lib/openapi_first/test/coverage/terminal_formatter.rb
index 412bf6bd..57d23fe7 100644
--- a/lib/openapi_first/test/coverage/terminal_formatter.rb
+++ b/lib/openapi_first/test/coverage/terminal_formatter.rb
@@ -30,8 +30,7 @@ def print(string)
         end
 
         def format_plan(plan)
-          filepath = plan.filepath
-          puts ['', "API validation coverage for #{filepath}: #{plan.coverage}%"]
+          puts ['', "API validation coverage for #{plan.api_identifier}: #{plan.coverage}%"]
           return if plan.done? && !verbose
 
           plan.routes.each do |route|
diff --git a/spec/definition_spec.rb b/spec/definition_spec.rb
index 57ce6897..3bfd824a 100644
--- a/spec/definition_spec.rb
+++ b/spec/definition_spec.rb
@@ -21,6 +21,47 @@ def build_request(path, method: 'GET')
     end
   end
 
+  describe '#key' do
+    context 'when filepath is available' do
+      it 'returns the filepath' do
+        definition = OpenapiFirst.parse({
+                                          'openapi' => '3.1.0',
+                                          'paths' => {}
+                                        }, filepath: '/path/to/openapi.yaml')
+
+        expect(definition.key).to eq('/path/to/openapi.yaml')
+      end
+    end
+
+    context 'when filepath is not available' do
+      it 'generates a key from info.title and info.version' do
+        definition = OpenapiFirst.parse({
+                                          'openapi' => '3.1.0',
+                                          'info' => {
+                                            'title' => 'Test API',
+                                            'version' => '1.0.0'
+                                          },
+                                          'paths' => {}
+                                        })
+        expect(definition.key).to eq('Test API @ 1.0.0')
+      end
+    end
+
+    context 'when the OpenAPI document is missing info.title or info.version' do
+      it 'raises an error' do
+        definition = OpenapiFirst.parse({
+                                          'openapi' => '3.1.0',
+                                          'info' => {
+                                            'title' => 'Test API'
+                                            # Missing version
+                                          },
+                                          'paths' => {}
+                                        })
+        expect { definition.key }.to raise_error(ArgumentError, /Cannot generate key/)
+      end
+    end
+  end
+
   describe '#paths' do
     it 'returns all paths' do
       definition = OpenapiFirst.load('./spec/data/petstore.yaml')
diff --git a/spec/openapi_first_spec.rb b/spec/openapi_first_spec.rb
index 55e97020..8cc26c6e 100644
--- a/spec/openapi_first_spec.rb
+++ b/spec/openapi_first_spec.rb
@@ -57,6 +57,13 @@
       expect(definition.paths).to include('/pets')
     end
 
+    it 'returns the same definition when a Definition object is passed in' do
+      original_definition = OpenapiFirst.load('./spec/data/petstore.yaml')
+      returned_definition = OpenapiFirst.load(original_definition)
+
+      expect(returned_definition).to be(original_definition)
+    end
+
     describe 'only option' do
       specify 'with empty filter' do
         definition = OpenapiFirst.load(spec_path, only: nil)
diff --git a/spec/test_spec.rb b/spec/test_spec.rb
index 5b5b38dc..218848f7 100644
--- a/spec/test_spec.rb
+++ b/spec/test_spec.rb
@@ -29,6 +29,48 @@
       expect(described_class[:mine].filepath).to eq('./examples/openapi.yaml')
     end
 
+    it 'can register a Definition object' do
+      definition = OpenapiFirst.load('./examples/openapi.yaml')
+      described_class.register(definition, as: :from_definition)
+      expect(described_class[:from_definition]).to eq(definition)
+    end
+
+    it 'uses filepath as key for Definition objects with filepath' do
+      # Register a definition with filepath and start tracking
+      definition = OpenapiFirst.load('./spec/data/dice.yaml')
+      described_class.register(definition, as: :with_filepath)
+      OpenapiFirst::Test::Coverage.start
+
+      # Verify the plan was registered with the filepath key
+      filepath = './spec/data/dice.yaml'
+      plan = OpenapiFirst::Test::Coverage.current_run[filepath]
+
+      expect(plan).not_to be_nil
+      expect(plan.filepath).to eq(filepath)
+      expect(plan.api_identifier).to eq(filepath)
+    end
+
+    it 'uses the definition key for Definition objects without filepath' do
+      # Create a definition without filepath
+      dice_hash = YAML.load_file('./spec/data/dice.yaml')
+      dice_hash['info'] = {
+        'title' => 'Dice API',
+        'version' => '1.0.0'
+      }
+      definition = OpenapiFirst.parse(dice_hash)
+
+      # Register and start tracking
+      described_class.register(definition, as: :without_filepath)
+      OpenapiFirst::Test::Coverage.start
+
+      expected_key = definition.key
+      plan = OpenapiFirst::Test::Coverage.current_run[expected_key]
+
+      # Verify the plan was registered with the definition key
+      expect(plan).to be_a(OpenapiFirst::Test::Coverage::Plan)
+      expect(plan.api_identifier).to eq(expected_key)
+    end
+
     it 'raises an error if the same API description is registered twice' do
       described_class.register('./examples/openapi.yaml')
       expect do