Cameo

A cameo has two meanings:

1. A brief appearance by a "name actor" (well-known actor) or celebrity, often appearing as himself or herself, in an episode of a TV series in which he or she is not a recurring character. In a series with a runtime of 40 minutes or more, the cut-off time for a cameo appearance is usually 120 seconds (two minutes) or less. In a series with a runtime of less than 40 minutes, the cutoff is usually 90 seconds or less.
2. An appearance of any actor who is hidden in shadow or by other means or heard only in voice-over in a live action fictional series. In this case, cameos may be long or brief, and cameo actors may be credited as leads, guest stars, co-stars or not at all. Notable TV cameos include multiple famous guest actors as callers on Dr. Frasier Crane's radio show in Frasier, or every appearance of Wilson in Home Improvement, or Ron Howard as the Narrator on Arrested Development.