Flying
Keyword abilities
An evasion ability — only flying or reach can block it.
How it works
Flying is an evasion ability. A creature with flying can only be blocked by creatures that also have flying or that have reach. It can still block any creature — flying limits who can block it, not who it can block.
Flying changes nothing about how much or what kind of damage the creature deals; it only governs blocking legality during the declare-blockers step.
Common confusions
“Can a creature with flying block a creature without flying?”
Yes. Flying only restricts what can block the flyer. A flyer can block anything, ground creatures included.
“A 1/1 flyer is blocked, then loses flying mid-combat.”
Blocks are locked in once declared. Losing flying after blocks are declared doesn't remove a legal block, and gaining it after doesn't let a blocker 'reach' an already-resolved declaration.
“Reach vs. flying.”
Reach lets a creature block flyers, but reach is not evasion — a creature with only reach can itself be blocked by anything.