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).
... terms of the coefficients a, b and c, which are assumed to be real (but see below for generalizations) with a being non-zero. These solutions are also called ...