Consonance is the pleasant sensation from certain combinations of notes or other sounds that are heard simultaneously. It is the opposite of dissonance, which is a harsh or unpleasant sensation from other combinations of notes or sounds.
Consonance and dissonance are a matter of degree, and some combinations are more consonant or more dissonant than others. Whether combinations are consonant or dissonant and to what extent is determined in large part by physics. For example, any two sounds whose frequencies are an octave apart have a particularly pleasing sensation when heard. But what is considered consonant and what is considered dissonant can also vary greatly according to the style of music and, moreover, they can change over time as music styles and tastes change.