Additional effect and action from a skill that cancels casting of usually an Active skill. All hard CC effects can interrupt (stun, knock-up, knockback, etc.). Some types of soft CCs can interrupt too like Silence and Blind. Some skills have interrupt as the only added effect.
NB: Achilles can still use his power while silenced but not while blinded. Hades cannot use his Ultimate whether he Silenced. when Hermes uses trickster on a random opponent, it may cancel their casting of a skill. Heroes like Argus and Tiresias have an interrupt on their 3rd skill on rotation.