Flying

Flying is a keyword ability that makes creatures without flying unable to block a creature with flying. It has been in Magic since the original Alpha set.

From the Comprehensive Rules:

• 502.4. Flying
  • 502.4a Flying is an evasion ability.
  • 502.4b A creature with flying can’t be blocked by creatures without flying. A creature with flying can block a creature with or without flying. (See rule 309, "Declare Blockers Step.")
  • 502.4c Multiple instances of flying on the same creature are redundant.