What is elevator music?

Elevator music is the music that is typically played while riding in an elevator in places such as a hotel. Generally, it is a slower jazz piece.

"Elevator music" is the general, mildly pejorative term for music that is written or
performed with the intention of having the music set the atmosphere or the mood
of a place, and be otherwise ignored and not call attention to itself. 

It's 'background music', 'dinner music' etc.  It's music in the gaseous state.

Real music has a beginning, a structure, a middle, a development, a melody,
some variations, a resolution, and an end. 'Elevator music' really doesn't need
any of these things.  It's built and performed in such a way that its effects on a
listener completely DON'T depend on when he comes in, how long he stays,
whether he tunes in or tunes out, and when he leaves.