based on custom rather than documentation; "an unwritten law"; "rites...so ancient that they well might have had their unwritten origins in Aurignacian times"- J.L.T.C.Spence
a note that alternates rapidly with another note a semitone above it
the articulation of a consonant (especially the consonant `r') with a rapid flutter of the tongue against the palate or uvula; "he pronounced his R's with a distinct trill"
pronounce with a trill, of the phoneme `r'; "Some speakers trill their r's"