[DOCS] Add EQL glossary def (#68938)

docs/reference/glossary.asciidoc

@@ -130,6 +130,14 @@ getting or searching for a document.
 // end::document-def[]
 --
 
+[[glossary-eql]]
+Event Query Language (EQL) ::
+// tag::eql-def[]
+A query language for event-based time series data, such as logs, metrics, and
+traces. EQL supports matching for event sequences. In the {security-app}, you
+use EQL to write event correlation rules. See {ref}/eql.html[EQL].
+// end::eql-def[]
+
 [[glossary-field]] field ::
 +
 --