docs: add glossary (zarr-developers#3767)

* docs: add glossary

* Add glossary

* Update docs/user-guide/glossary.md

Co-authored-by: Davis Bennett <davis.v.bennett@gmail.com>

* Add caveat

---------

Co-authored-by: Davis Bennett <davis.v.bennett@gmail.com>

Author: maxrjones

docs/user-guide/glossary.md

@@ -0,0 +1,110 @@
+# Glossary
+
+This page defines key terms used throughout the zarr-python documentation and API.
+
+## Array Structure
+
+### Array
+
+An N-dimensional typed array stored in a Zarr [store](#store). An array's
+[metadata](#metadata) defines its shape, data type, chunk layout, and codecs.
+
+### Chunk
+
+The fundamental unit of data in a Zarr array. An array is divided into chunks
+along each dimension according to the [chunk grid](#chunk-grid), which is currently
+part of Zarr's private API. Each chunk is independently compressed and encoded
+through the array's [codec](#codec) pipeline.
+
+When [sharding](#shard) is used, "chunk" refers to the inner chunks within each
+shard, because those are the compressible units. The chunks are the smallest units
+that can be read independently.
+
+!!! warning "Convention specific to zarr-python"
+    The use of "chunk" to mean the inner sub-chunk within a shard is a convention
+    adopted by zarr-python's `Array` API. In the Zarr V3 specification and in other
+    Zarr implementations, "chunk" may refer to the top-level grid cells (which
+    zarr-python calls "shards" when the sharding codec is used). Be aware of this
+    distinction when working across libraries.
+
+**API**: [`Array.chunks`][zarr.Array.chunks] returns the chunk shape. When
+sharding is used, this is the inner chunk shape.
+
+### Chunk Grid
+
+The partitioning of an array's elements into [chunks](#chunk). In Zarr V3, the
+chunk grid is defined in the array [metadata](#metadata) and determines the
+boundaries of each storage object.
+
+When sharding is used, the chunk grid defines the [shard](#shard) boundaries,
+not the inner chunk boundaries. The inner chunk shape is defined within the
+[sharding codec](#shard).
+
+**API**: The `chunk_grid` field in array metadata contains the storage-level
+grid.
+
+### Shard
+
+A storage object that contains one or more [chunks](#chunk). Sharding reduces the
+number of objects in a [store](#store) by grouping chunks together, which
+improves performance on file systems and object storage.
+
+Within each shard, chunks are compressed independently and can be read
+individually. However, writing requires updating the full shard for consistency,
+making shards the unit of writing and chunks the unit of reading.
+
+Sharding is implemented as a [codec](#codec) (the sharding indexed codec).
+When sharding is used:
+
+- The [chunk grid](#chunk-grid) in metadata defines the shard boundaries
+- The sharding codec's `chunk_shape` defines the inner chunk size
+- Each shard contains `shard_shape / chunk_shape` chunks per dimension
+
+**API**: [`Array.shards`][zarr.Array.shards] returns the shard shape, or `None`
+if sharding is not used. [`Array.chunks`][zarr.Array.chunks] returns the inner
+chunk shape.
+
+## Storage
+
+### Store
+
+A key-value storage backend that holds Zarr data and metadata. Stores implement
+the [`zarr.abc.store.Store`][] interface. Examples include local file systems,
+cloud object storage (S3, GCS, Azure), zip files, and in-memory dictionaries.
+
+Each [chunk](#chunk) or [shard](#shard) is stored as a single value (object or
+file) in the store, addressed by a key derived from its grid coordinates.
+
+### Metadata
+
+The JSON document (`zarr.json`) that describes an [array](#array) or group. For
+arrays, metadata includes the shape, data type, [chunk grid](#chunk-grid), fill
+value, and [codec](#codec) pipeline. Metadata is stored alongside the data in
+the [store](#store). Zarr-Python does not yet expose its internal metadata
+representation as part of its public API.
+
+## Codecs
+
+### Codec
+
+A transformation applied to array data during reading and writing. Codecs are
+chained into a pipeline and come in three types:
+
+- **Array-to-array**: Transforms like transpose that rearrange array elements
+- **Array-to-bytes**: Serialization that converts an array to a byte sequence
+  (exactly one required)
+- **Bytes-to-bytes**: Compression or checksums applied to the serialized bytes
+
+The [sharding indexed codec](#shard) is a special array-to-bytes codec that
+groups multiple [chunks](#chunk) into a single storage object.
+
+## API Properties
+
+The following properties are available on [`zarr.Array`][]:
+
+| Property | Description |
+|----------|-------------|
+| `.chunks` | Chunk shape — the inner chunk shape when sharding is used |
+| `.shards` | Shard shape, or `None` if no sharding |
+| `.nchunks` | Total number of independently compressible units across the array |
+| `.cdata_shape` | Number of independently compressible units per dimension |

docs/user-guide/index.md

@@ -35,6 +35,10 @@ Take your skills to the next level:
 - **[Extending](extending.md)** - Extend functionality with custom code
 - **[Consolidated Metadata](consolidated_metadata.md)** - Advanced metadata management
 
+## Reference
+
+- **[Glossary](glossary.md)** - Definitions of key terms (chunks, shards, codecs, etc.)
+
 ## Need Help?
 
 - Browse the [API Reference](../api/zarr/index.md) for detailed function documentation

mkdocs.yml

@@ -27,6 +27,7 @@ nav:
       - user-guide/gpu.md
       - user-guide/consolidated_metadata.md
       - user-guide/experimental.md
+      - user-guide/glossary.md
   - Examples:
       - user-guide/examples/custom_dtype.md
   - API Reference: