Merge pull request #51411 from jldohmann/new-glossary-term

add term boot image

Author: jldohmann

contributing_to_docs/term_glossary.adoc

@@ -100,6 +100,13 @@ action. It consists of _identity_ and _action_.
 
 == B
 
+''''
+=== boot image
+
+Usage: boot image(s)
+
+* A boot image is a disk image that contains a bootable operating system (OS) and all the configuration settings for the OS, such as drivers.
+
 ''''
 === build