Either of a pair of symbols, [], used for various purposes, e.g. to enclose text inserted or replaced by an editor, and to indicate various mathematical operations.
1905, Royal Irish Academy, Proceedings of the Royal Irish Academy, page 33:
We must, in short, employ a special bracket for operative relations ; and the square bracket is the most convenient. Hence [a + bβ] x denotes that a + bβ operates on x, and [φ]n denotes that φ operates on itself n - 1 times.