What is a chromatic scale in musical terms?

A chromatic scale is a scale where each note of it is separated by a semiton - that is - it includes all the notes that exist.

C1, C1#, D1, D1#, E1, F1, F1# ... B1, C2, C2#, D2... etc

For instance, in a keyboard that would mean you'd play all the white and black keys sucessively.