The term 'chromatic', from the Greek chromatikos meaning 'colored', is used in melodic and harmonic analysis to refer to notes which don't occur in the scale of the key of the passage. Such notes are marked with accidentals and hence the chords in which they occur are also termed chromatic. Use of the term depends on the definition of the key of the passage.
The chromatic scale divides an octave into twelve semitones – 7 naturals and 5 non-naturals (all the white and black notes on the keyboard from middle C to the C above it, for example) – as opposed to the diatonic major and minor scales. Chromatic chords employ notes foreign to the diatonic scale of the prevailing key in a musical passage.
A chromatic passing note is one that bridges between two notes of the diatonic scale and "passes through" as the melodic line moves from one of those notes to the other.
When songs use the chromatic scale it often lends an exotic or sinister feel to the music.
The history of Western music through the early twentieth century reveals a progression of increasing chromaticism or chromatic harmony. Chromaticism is associated with the later stages of Romantic music and the onset of atonality (see atonal): composers felt increasingly free to bring most or all of the twelve chromatic notes into play, bending or abandoning the rules of tonal harmony.