Oscillator sync is a feature in some synthesizers with two or more VCOs (or digital emulations of VCOs). One oscillator will restart the period of another oscillator, so that they will have the same base frequency. The timbre can be altered on the synched oscillator by varying its frequency input.
A synched oscillator that resets the other oscillator(s) is called the master, and any synched oscillator that is reset by another oscillator is called a slave.
There are two common forms of oscillator sync which appear on synthesizers: Hard Sync and Soft Sync. Soft Sync is a term used for a variety of mechanisms. Other sync mechanisms exist which are also discussed.
Read more about Oscillator Sync: Hard Sync, Soft Sync, Digital Implementation Aspects, Sync-based Architectures