docs: add glossary of terms to general reference

Add a glossary defining key terms (Option, flag, command line
application, command line utility, terminal, etc.) as they are
used throughout the Click documentation. Uses MyST cross-references
to link terms to their detailed documentation pages.

Fixes #3077

Author: Jiahao Ren

docs/glossary.md

@@ -0,0 +1,61 @@
+# Glossary
+
+```{currentmodule} click
+```
+
+This glossary defines key terms as they are used throughout the Click
+documentation. Having a shared vocabulary helps keep the docs internally
+consistent and reduces ambiguity for new users.
+
+## Terms
+
+**Application**
+:   A complete command line program built with Click. May consist of a
+    single command or a tree of commands and groups. Used interchangeably
+    with "command line application" in this documentation.
+
+**Argument**
+:   A positional parameter passed to a command. Arguments are
+    required by default and are identified by their position on the
+    command line, not by name. See {doc}`arguments`.
+
+**Command**
+:   A single callable unit in a Click application. Created with
+    {func}`~click.command`. A command can accept
+    {class}`~click.Option` and {class}`~click.Argument` parameters.
+    See {doc}`commands`.
+
+**Command line application**
+:   A program that is operated by typing text commands into a terminal.
+    In Click, this is any program built using the Click framework.
+    Used interchangeably with "application".
+
+**Command line utility**
+:   A command line application designed for a specific task, often
+    without a persistent interactive session. Examples include ``ls``,
+    ``grep``, and ``curl``. In Click documentation, "application" is
+    preferred for clarity.
+
+**Flag**
+:   An option that takes no value, acting as a boolean switch. Created
+    by passing `is_flag=True` to {func}`~click.option`. Example:
+    ``--verbose``. See {ref}`is-flag`.
+
+**Group**
+:   A command that contains subcommands. Created with
+    {func}`~click.group`. Groups enable nested CLI structures where
+    the first argument selects a subcommand. See {doc}`commands-and-groups`.
+
+**Option**
+:   A named parameter passed to a command using ``--name value`` or
+    ``--name=value`` syntax. Options are optional by default. See
+    {doc}`options`.
+
+**Parameter**
+:   A generic term covering both {term}`options <Option>` and
+    {term}`arguments <Argument>`. See {doc}`parameters`.
+
+**Terminal**
+:   The text-based interface where a command line application is
+    executed. Also called a "console", "shell", or "command prompt".
+    Click uses "terminal" throughout its documentation.

docs/index.rst

@@ -118,6 +118,7 @@ General Reference
    command-line-reference
    unicode-support
    wincmd
+   glossary
 
 API Reference
 -------------------