In the fictional Star Trek universe, the Trill are two symbiotic races of aliens; see Trill (Star Trek).
For cats, a trill is a type of sound made by a Maine coon cat.
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In phonetics, a trill is a consonantal sound produced by vibrations between the articulator and the place of articulation.
Whereas with a flap (or tap), a specific gesture is used to strike the active articulator against the passive one, in the case of a trill the articulator is held in place, where the airstream causes it to vibrate.
Epiglottal consonants are often allophonically trilled, and in some languages the trill is the primary realization of the consonant.
The trill is a musical ornament consisting of a rapid alternation between two notes (compare tremolo).
These factors, together with the overall rate of the trill and whether that rate is constant or variable, can only be determined by considering the context in which the trill appears, and is usually to a large degree a matter of opinion with no single "right" way of executing the ornament.
For instance, it is relatively easy to produce a trill on the piano, but on the trombone it is virtually impossible because of the slide mechanism.