The chromatic scale is a scale of twelve notes where the difference between each two adjacent notes is approximately the same.
That is, the chromatic scale is the sequence of the familiar twelve notes used in contemporary music. The chromatic scale starting in C, for example, is the scale C, C#/Db, D, D#/Eb, E, F, F#/Gb, G, G#/Ab, A, A#/Bb, B.
The chromatic scale is rarely used in its entirety, but heptatonic scales and pentatonic scales built from notes on the chromatic scale are the most common scales in contemporary Western music.
The difference between each two adjacent notes is called a chromatic semitone. If the chromatic scale is equal tempered, then all semitones are the same (the ratio C / C# is the same as the ratio D / Db, and so on; each semitone is then equal to 2^(1/12) or 100 cents). If the chromatic scale is just tempered, then the semitones are not necessarily the same.