Wiktionary
syncword
n. (context computing English) A short marker used to synchronize a transmission by indicating the end of the header information and the start of the data.
Wikipedia
Syncword
In computer networks, a syncword, sync character, sync sequence or preamble is used to synchronize a data transmission by indicating the end of header information and the start of data. The syncword is a known sequence of data used to identify the start of a frame, and is also called reference signal or midamble in wireless communications.
Prefix codes allow unambiguous identification of synchronization sequences and may serve as self-synchronizing code.