Alternative

A situation which allows a mutually exclusive choice between two or more possibilities; a choice between two or more possibilities.

One of several mutually exclusive things which can be chosen.

The remaining option; something available after other possibilities have been exhausted.

Alternative rock

Relating to a choice between two or more possibilities.

"an alternative proposition"

Other; different from something else.

Not traditional, outside the mainstream, underground.

Alternate, reciprocal.