Stories We Like: A Guide to the Comma
1929, shortened form of synchronization (see synchronize). Originally in reference to soundtracks and pictures in the movies. Sense of "be in agreement, coincide" first recorded 1961 in in sync.
Synchronism; synchronization (1929+)
To synchronize: Let's sync our plans, okay? (1950s+)
in sync, lip-sync, out of sync