An instrument is a concrete or abstract tool intended for a purpose other than mechanical work, in particular a refined one.
Musical instruments are devices designed to produce music, frequently by plucking or striking a string or bell or drum surface, or by forcing air to resonate (see wind instruments).
... is now in a form in which we can conveniently complete the square[?]. To "complete the square" is to add a constant (i.e., in this case, a quantity that does not ...