Flanking
From M:TG Archive, a library of Magic: The Gathering information
Flanking is a keyword ability introduced in the Mirage block also present in the Time Spiral block.
Flanking is a triggered ability that triggers during the declare blockers step. "Flanking" means "Whenever this creature becomes blocked by a creature without flanking, the blocking creature gets -1/-1 until end of turn."
Multiple instances of flanking trigger separately.
