Reach



Reach is a static ability that means the affected creature can block creatures with flying.

Reach is a very old mechanic, and has existed at since Alpha's Giant Spider, although it did not have a keyword at the time and was simply spelled out in the rules text.

Example
Amy attacks with a Griffin Sentinel and Norman controls a Giant Spider, even though it doesn't have flying, the Giant Spider can still legally block the Griffin Sentinel, as it has reach.

Comprehensive Rules
702.17. Reach


 * 702.17a. Reach is a static ability.


 * 702.17b. A creature with flying can't be blocked except by creatures with flying and/or reach. (See rule 509, "Declare Blockers Step," and rule 702.9, "Flying.")


 * 702.17c. Multiple instances of reach on the same creature are redundant.