Possibly one of the toughest questions in the world to answer is, "What is music?" There is no concrete, agreed upon definition of what music actually is. One person's definition of what constitutes music might be completely different from another's.
The Oxford Dictionary defines music as:
Vocal or instrumental sounds (or both) combined in such a way as to produce beauty of form, harmony, and expression of emotion
A common description of music is that it's "organized sound." What is the difference, however, between noise and music. What one might perceive as noise another might perceive as music. A scientific way to differentiate is by recognizing patterns in the soundwaves and frequences. Noise occurs when soundwaves and frequencies are created with randomness and no distinct pattern. When music is defined as "organized sound," it means that the soundwaves and frequences display a pattern. The act of combining pure tones together to produce a complex waveform is called additive synthesis. A few other terms to make the distinction are "patterned sounds" or "ordered sounds" vs "unordered sounds."
Here is a simplistic visual of the distinction:
If you are a real physics geek and want to take a look at the distinction in depth, check out this extremely comprehensive overview from physics.info.
There is also an interesting Wikipedia page dedicated to the Definition of Music. Not "Music" itself, but actual question of how to define music.