Superposition

Instruments are designed so as to reinforce certain harmonics vital to that instrument's tone.  For this reason, a violin sounds nothing like a piano and it is relatively easy to identify instruments contributing to an orchestral piece even if they are playing the same note.  This is because the sound waves produced by all instruments are very different.

Sound waves are created by adding together the harmonic waves produced by the instrument which are unique to each individual instrument.  This principle of adding together waves is known as superposition.