Trill is a tricky one.

I am offering an abbreviated version of what I found at

  1. A repetitive succession of rapidly alternating sounds.
  2. (Phonetics): A trill can be executed while singing, talking or whistling.
  3. (Music): An alternation of two rapid tones.
  4. To flow into a stream.

What all these definitions appear to have in common is that they posit an endless motion of sound or water that is repeated cyclically.