500.3a.

An evasion ability is an ability an attacking creature has that restricts what can block it. Evasion abilities are static abilities that modify the declare blockers step of the combat phase. If a creature gains or loses an evasion ability after a legal block has been declared, it doesn't affect that block. Evasion abilities are cumulative.

Example: A creature with flying and shadow can't be blocked by a creature with flying but without shadow.