DOCS-13830 BACKPORT (#1714)

Author: Dave Cuthbert

source/core/replica-set-oplog.txt

@@ -151,6 +151,11 @@ the value of the corresponding configuration file setting or
 command line option to persist those changes through a server
 restart.
 
+Oplog Window
+~~~~~~~~~~~~
+
+.. include:: /includes/replication/fact-oplog-window-definition.rst
+
 Workloads that Might Require a Larger Oplog Size
 ------------------------------------------------
 

source/includes/replication/fact-oplog-window-definition.rst

@@ -0,0 +1,5 @@
+:term:`oplog` entries are time-stamped. The oplog window is the time
+difference between the newest and the oldest timestamps in the
+``oplog``. If a secondary node loses connection with the primary, it
+can only use :ref:`replication <replication>` to sync up again if the
+connection is restored within the oplog window. 

source/reference/glossary.txt

@@ -648,6 +648,9 @@ Glossary
       basic mechanism enabling :term:`replication` in MongoDB.
       See :doc:`/core/replica-set-oplog`.
 
+   oplog window
+      .. include:: /includes/replication/fact-oplog-window-definition.rst
+
    optime
       .. versionchanged:: 3.2