Add glossary to documentation

Add a glossary page to the Reference section with definitions of
Vitess-specific terms, components, and concepts. Addresses #1982.

The glossary covers:
- Core components (VTGate, VTTablet, vtctld, VTOrc, etc.)
- Sharding concepts (keyspace, shard, vindex, keyspace ID)
- VReplication workflows (MoveTables, Reshard, Materialize)
- Tablet types and states
- Topology and infrastructure terms

Terms are organized alphabetically with cross-references where
appropriate.

Author: promptless[bot]

content/en/docs/23.0/reference/glossary.md

@@ -0,0 +1,207 @@
+---
+title: Glossary
+description: Definitions of terms used in Vitess documentation
+weight: 1
+---
+
+This document defines terms used throughout Vitess documentation, command-line tools, and source code.
+
+## Backup
+
+A tablet state indicating the tablet is currently taking a backup of its data.
+
+## Cell
+
+A data center, availability zone, or other failure domain. Vitess uses cells to isolate failures and route queries to geographically appropriate tablets.
+
+## Consistent Lookup Vindex
+
+A [lookup vindex](#lookup-vindex) that maintains consistency without requiring two-phase commit.
+
+## Drained
+
+A tablet state reserved for Vitess background operations. Drained tablets do not serve queries.
+
+## Functional Vindex
+
+A [vindex](#vindex) that computes the [keyspace ID](#keyspace-id) using an algorithmic mapping such as a hash or identity function, without consulting external data.
+
+## Global Topology
+
+The portion of the [topology service](#topology-service) that stores cluster-wide metadata shared across all [cells](#cell).
+
+## GTID Position
+
+Global Transaction Identifier position. Marks a specific point in the MySQL binary log for replication tracking.
+
+## Keyrange
+
+The range of [keyspace IDs](#keyspace-id) that a [shard](#shard) is responsible for. Expressed in hexadecimal notation like `-80` (up to 0x80) or `80-` (from 0x80 onward).
+
+## Keyspace
+
+A logical database that maps to one or more MySQL databases. If using [sharding](#shard), a keyspace spans multiple databases; otherwise it maps to a single MySQL database. Applications interact with keyspaces as if they were single databases.
+
+## Keyspace ID
+
+A computed value that determines which [shard](#shard) contains a given row. The [primary vindex](#primary-vindex) maps a column value to a keyspace ID.
+
+## Local Topology
+
+The portion of the [topology service](#topology-service) that stores metadata specific to a single [cell](#cell).
+
+## Lookup Vindex
+
+A [vindex](#vindex) that uses a lookup table to map column values to [keyspace IDs](#keyspace-id). Enables routing queries on columns other than the sharding key.
+
+## Materialize
+
+A VReplication workflow that creates and maintains materialized views or realtime rollups across [shards](#shard).
+
+## MoveTables
+
+A VReplication workflow that migrates tables from one [keyspace](#keyspace) to another.
+
+## mysqlctl
+
+A command-line utility for managing the lifecycle of local MySQL instances used by Vitess.
+
+## Online DDL
+
+Non-blocking schema changes that allow tables to remain available during migrations. Vitess supports multiple strategies including its native approach, gh-ost, and pt-osc.
+
+## Primary
+
+The tablet type for the MySQL primary of a [shard](#shard). Handles all write operations. Formerly called "master."
+
+## Primary Vindex
+
+The [vindex](#vindex) that determines row-to-shard mapping. Equivalent to the sharding key.
+
+## Rdonly
+
+A tablet type for read-only replicas that are not eligible for promotion to primary. Used for batch jobs, analytics, and other background operations.
+
+## Reparenting
+
+The process of changing which [tablet](#tablet) serves as the [primary](#primary) for a [shard](#shard).
+
+## Replica
+
+A tablet type for MySQL replicas that are eligible for promotion to [primary](#primary).
+
+## Replication Graph
+
+Metadata that tracks MySQL primary-replica relationships within the cluster.
+
+## Reshard
+
+A VReplication workflow that splits or merges [shards](#shard) to scale horizontally.
+
+## Restore
+
+A tablet state indicating the tablet is restoring data from a backup.
+
+## Routing Rules
+
+Configuration that overrides default query routing during migrations, allowing gradual traffic shifts between [keyspaces](#keyspace).
+
+## Secondary Vindex
+
+A [vindex](#vindex) that provides additional query routing optimization beyond the [primary vindex](#primary-vindex).
+
+## Shard
+
+A horizontal partition of a [keyspace](#keyspace). Each shard contains a subset of the keyspace's data based on [keyrange](#keyrange). An unsharded keyspace has a single shard named `0` or `-`.
+
+## SwitchTraffic
+
+A VReplication operation that redirects query traffic from a source to a target during workflows like [MoveTables](#movetables) or [Reshard](#reshard).
+
+## Tablet
+
+A combination of a `vttablet` process and its associated MySQL instance. Tablets are the basic unit of data storage and query serving in Vitess.
+
+## Tablet Alias
+
+A unique identifier for a [tablet](#tablet) in the format `cell-uid`, such as `zone1-100`.
+
+## Topology Service
+
+A metadata store (backed by etcd or ZooKeeper) that holds cluster configuration, [replication graph](#replication-graph), and serving state. Often abbreviated as "topo."
+
+## Unsharded
+
+A [keyspace](#keyspace) with a single [shard](#shard), containing all data without horizontal partitioning.
+
+## VDiff
+
+A VReplication tool that verifies data consistency between source and target during migrations.
+
+## VEXPLAIN
+
+A SQL command that shows how Vitess will execute a query across [shards](#shard), including the generated MySQL queries.
+
+## Vindex
+
+Virtual index. A mechanism that maps column values to [keyspace IDs](#keyspace-id) for query routing. Types include [primary](#primary-vindex), [secondary](#secondary-vindex), [lookup](#lookup-vindex), and [functional](#functional-vindex).
+
+## VReplication
+
+Vitess's streaming replication mechanism for data movement operations like [MoveTables](#movetables), [Reshard](#reshard), and [Materialize](#materialize).
+
+## VSchema
+
+Virtual schema. Configuration that defines how a [keyspace](#keyspace) is sharded, including [vindexes](#vindex), table definitions, and routing rules.
+
+## VStream
+
+A change notification service on [VTGate](#vtgate) that streams row changes to clients.
+
+## VStreamer
+
+The component on [VTTablet](#vttablet) that streams MySQL binary log events for [VReplication](#vreplication) and [VStream](#vstream).
+
+## vtbackup
+
+A standalone utility for taking backups of MySQL data managed by Vitess.
+
+## vtcombo
+
+A single-process test environment that runs all Vitess components together. Useful for local development and testing.
+
+## VTAdmin
+
+A web-based administrative interface for managing and monitoring Vitess clusters.
+
+## vtctl
+
+The legacy command-line administration tool for Vitess. Deprecated in favor of [vtctldclient](#vtctldclient).
+
+## vtctld
+
+The Vitess control daemon. A server process that executes cluster management commands issued by [vtctldclient](#vtctldclient).
+
+## vtctldclient
+
+The current command-line client for Vitess cluster management. Communicates with [vtctld](#vtctld) to execute administrative operations.
+
+## VTEXPLAIN
+
+A command-line tool for offline query analysis that shows how Vitess would route and execute queries without connecting to a live cluster.
+
+## VTGate
+
+A lightweight, stateless proxy that routes queries to the appropriate [tablets](#tablet). Applications connect to VTGate rather than directly to MySQL.
+
+## VTOrc
+
+Vitess Orchestrator. Provides automated failover, recovery, and tablet management for Vitess clusters.
+
+## VTTablet
+
+The Vitess process that manages a MySQL instance, handling query serving, replication, and schema management. Together with MySQL, it forms a [tablet](#tablet).
+
+## vttestserver
+
+A lightweight Vitess server for local development and testing that simulates a Vitess cluster.