ref-manual: add description for the "sysroot" term

This term is used throughout the manual but is not properly
introduced anywhere.

(From yocto-docs rev: 51426a0762a5988aa835e065f5258fd7ac474c52)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/terms.rst

@@ -383,6 +383,31 @@ universal, the list includes them just in case:
      ":ref:`overview-manual/development-environment:repositories, tags, and branches`"
      section in the Yocto Project Overview and Concepts Manual.
 
+   :term:`Sysroot`
+      When cross-compiling, the target file system may be differently laid
+      out and contain different things compared to the host system. The concept
+      of a *sysroot* is directory which looks like the target filesystem and
+      can be used to cross-compile against.
+
+      In the context of cross-compiling toolchains, a *sysroot*
+      typically contains C library and kernel headers, plus the
+      compiled binaries for the C library. A *multilib toolchain*
+      can contain multiple variants of the C library binaries,
+      each compiled for a target instruction set (such as ``armv5``,
+      ``armv7`` and ``armv8``), and possibly optimized for a specific CPU core.
+
+      In the more specific context of the OpenEmbedded build System and
+      of the Yocto Project, each recipe has two sysroots:
+
+      -  A *target sysroot* contains all the **target** libraries and headers
+         needed to build the recipe.
+
+      -  A *native sysroot* contains all the **host** files and executables
+         needed to build the recipe.
+
+      See the :term:`SYSROOT_* <SYSROOT_DESTDIR>` variables controlling
+      how sysroots are created and stored.
+
    :term:`Task`
       A per-recipe unit of execution for BitBake (e.g.
       :ref:`ref-tasks-compile`,