[DOCS] Add 'feature state' definition to glossary (#1882)

Closes elastic/elasticsearch#80050

Relates to elastic/elasticsearch#79675, elastic/elasticsearch#79081

docs/en/glossary/glossary.asciidoc

@@ -437,6 +437,15 @@ and
 {ml-docs}/ml-dfa-classification.html#dfa-classification-feature-importance[{classification-cap} feature importance].
 //Source: Machine learning
 
+[[glossary-feature-state]] feature state::
+The indices and data streams used to store configurations, history, and other
+data for an Elastic feature, such as {es} security or {kib}. A feature state
+typically includes one or more <<glossary-system-index,system indices or data
+streams>>. It may also include regular indices and data streams used by the
+feature. You can use <<glossary-snapshot,snapshots>> to back up and restore
+feature states. See {ref}/snapshot-restore.html#feature-state[feature states].
+//Source: Elasticsearch
+
 [[glossary-field]] field::
 . Key-value pair in a <<glossary-document,document>>. See
 {ref}/mapping.html[Mapping].