a vibrating or quivering sound in which one rapidly alternates between two notes, especially when singing or playing a musical instrument, or something resembling such a sound;
e.g. Her voice made a trill when she sang the highest note of the song. I could hear the trill of the birds outside my window.
(verb)
to produce a vibrating or quivering sound by rapidly alternating between two notes, especially when singing or playing a musical instrument;
e.g. You must trill the long notes when playing this piece on the violin.