Historical Rules · 1 October 2009
Rule 702.22a
Official source text from the 1 October 2009 Rules release
Flanking is a triggered ability that triggers during the declare blockers step. (See rule 509, "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."