I know there is a term for a number that is the power of 2, such as 8, 32, 128, 4096 -- but it slipped my mind.
A "power of two" seems like a good name.
Although 'exponent' has been suggested, it is not correct
The relationship is quite simple, though:
In programming, especially graphics programming (textures), powers of two are sometimes referred to as POT, and non-powers of two as NPOT.
But obviously the word "pot" has strong connotations, and so this term can be used only in such a context unambiguously. Anywhere else, "power of two" is a much better choice.