n. & v.
n. 1 a quavering or vibratory sound, esp. a rapid alternation of sung or played notes. 2 a bird's warbling sound. 3 the pronunciation of r with a vibration of the tongue.
v. 1 intr. produce a trill. 2 tr. warble (a song) or pronounce (r etc.) with a trill.
[ It. trillo (n.), trillare (v.) ]