Fig 1. The hand sign for La (pitch of A) in solfege.
Fig 2. The notes of A in different octaves on the stave, in scientific pitch notation, and on a standard piano keyboard, with frequencies shown in hertz.
Fig 3. The chord of A major in open position on a guitar.
A is the note which in scientific pitch notation is written A4 (the A immediately below middle C) has a pitch of 440 Hz (oscillations per second) and is used as a standard tuning pitch for most Western music (see A440). Any octave transposition of this pitch is also an A. See Fig 2.
A0 is the lowest note on an 88-note piano. The fifth string of a guitar in standard tuning is tuned to A2 (110 Hz).
A may also refer to the key of A major and to a chord of A. For example, a chord of A major might be referred to simply as an "A". Fig 3 shows the chord of A major played on a guitar in open position.