Historical Rules · 18 June 2021
Rule 702.25a
Official source text from the 18 June 2021 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.”