Sync character
From Computer History Wiki
A sync character (short for synchronization character) is a special character used on a synchronous serial line to keep the clocks at both ends in synchronization. It is sent when there is no data ready to be sent over the line. If a line is idle, usually several sync characters are sent to allow the receiver to work out where the character boundaries are, before sending the special character(s) which mark the start of a frame.
