A trill is a musical ornament made by the more or less rapid alternation of a note and the note above, in the Classical period generally starting on the latter.

1640s, from Italian trillio, triglio "a quavering or warbling in singing," probably ultimately of imitative origin. The verb is 1660s, from Italian trillare "to quaver, trill." Rel [..]

Rapid alternation between notes that are a half tone or whole tone apart.

An ornament that consists of rapid alternation between one tone and another tone either a step or a semitone away from the first tone. 

  A trill ornaments a note, and is a rapid alternation between two pitches, usually a major or minor second above the note. The letters tr

(n) a note that alternates rapidly with another note a semitone above it(n) the articulation of a consonant (especially the consonant `r') with a rapid flutter of the tongue against the palate or [..]

The name of a manner of articulation. A trill consists of a series of rapid closures between two articulators. Trills for which the IPA provides symbols: bilabial, ʙ, alveolar r and uvular ʀ. The imag [..]

a fluttering sound that alternates rapidly with another note

Rapid alternation of two conjunct notes.

Musical ornament that consists of two notes a half step or a whole step apart played in rapid alternation.

Rapid alternation between notes that are a half tone or whole tone apart.

A trill is a type of ornament. It sounds like a fast warble between a note and the next note above or below it.

A rapid movement between two notes.

an “ornament” consisting of the more or less rapid alternation between two pitches in which the trilling note is one step above the written note. It is usually indicated with an abbreviation (tr) and/ [..]

An alternating pair of notes repeated many times. Trills can be played as grace notes leading into main melodic note, or as their own separate stand-alone part of the melody.

a high-pitched sound that is repeated rapidly. tropical