DOCS-13830 BACKPORT (#1715)

Author: Dave Cuthbert

source/core/replica-set-oplog.txt

@@ -108,6 +108,11 @@ oplog dynamically without restarting the :binary:`~bin.mongod` process.
 
    .. include:: /includes/fact-oplog-size.rst
 
+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

@@ -628,6 +628,9 @@ 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