The glossary doesn't include entries for the term "sync" in replica set context.

Additionally, it may be worthwhile to go through existing replica sets documentation to clarify/disambiguate terminology between initial sync (sometimes called "cloning" in the historical documentation) versus the mostly-not-ever-clearly-named oplog sync operation. If there are any cases where initial sync and oplog sync differ about certain behaviors (e.g., how they decide whom to sync from), it'd be good to clarify that sort of thing.