What is a trill in piano music?

In piano music, a trill is a rapid alternation between two adjacent notes. It is notated by placing the letters "tr" above the note, followed by a wavy line. The trill should be played with the fingers alternately striking the keys, starting on the upper note. Trills can be performed at different speeds and dynamics, depending on the musical context. They are often used to add ornamentation or flourish to a piece of music.