The trill (or shake, as it was known from the 16th until the early 20th century) is a musical ornament consisting of a rapid alternation between two adjacent notes, usually a semitone or tone apart, which can be identified with the context of the trill (compare mordent and tremolo). It is sometimes referred to by the German Triller, the Italian trillo, the French trille or the Spanish trino. A cadential trill is a trill associated with each cadence. A groppo or gruppo is a specific type of cadential trill which alternates with the auxiliary note directly above it and ends with a musical turn as additional ornamentation.

A trill provides rhythmic interest, melodic interest, and—through dissonance—harmonic interest. Sometimes it is expected that the trill will end with a turn (by sounding the note below rather than the note above the principal note, immediately before the last sounding of the principal note), or some other variation. Such variations are often marked with a few appoggiaturas following the note bearing the trill indication.

In most modern musical notation, a trill is generally indicated with the letters tr (or sometimes simply t) above the trilled note. This has sometimes been followed by a wavy line, and sometimes, in the baroque and early classical periods, the wavy line was used on its own. In those times the symbol was known as a chevron. The following two notations are equivalent:

Both the "tr" and the wavy line are necessary for clarity when the trill is expected to be applied to more than one note (or to tied notes). Also, when attached to a single notehead in one part that corresponds to smaller note values in another part, it leaves no room for doubt if both the letter and line are used.

The usual way of executing a trill, known as a diatonic trill, is to rapidly alternate between the written note and the one directly above it in the given scale (unless the trill symbol is modified by an accidental, understood to apply to the added note above; this is a chromatic trill).

This is an alternative trill:

These examples are approximations of how trills may be executed. In many cases, the rate of the trill does not remain constant as indicated here, but starts slower and increases. Whether it is played this way or not is largely a matter of taste.

The number of alternations between the notes played in a trill can vary according to the length of the notated note. At slower tempos, a written note lasts longer, meaning more notes can be played in the trill applied to it; but at fast tempo and with a short note, a trill may be reduced to nothing more than the indicated note, the note above it, and the indicated note again, in which case it resembles an upper mordent.