It has become synonymous with slide though when the manufactureres started calling their positive films [something]chrome, their negative films [something]color.
So it is after Kodachrome, Fujichrome, Agfachrome, etc. (vs Kodacolor, Fujicolor, Agfacolor, etc.)
I don't think that's correct.
Originally Posted by Ian Grant
It was because the thingy produced colour pictures, rather than monochrome.
Monochrome film that gave/gives a full chromatic response also exist, and the fact that they produce a full chromatic response is denoted by the prefix "pan".