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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.