Definitions of trill:

• noun:   a note that alternates rapidly with another note a semitone above it
• verb:   pronounce with a trill, of the phoneme `r'

  Example: "Some speakers trill their r's"

• verb:   sing or play with trills, alternating with the half note above or below
• name:  A surname (very rare: popularity rank in the U.S.: #67026)