CHROMATIC , a term meaning "coloured," chiefly used in science, particularly in the expression "chromatic aberration" or "dispersion" (see Aberration). In Greek music was one of three divisions - diatonic, chromatic and enharmonic - of the tetrachord. Like the Latin color, was often used of ornaments and embellishments, and particularly of the modification of the three genera of the tetrachord. The chromatic, being subject to three such modifications, was regarded as particularly "coloured." To the Greeks chromatic music was sweet and plaintive. From a supposed resemblance to the notes of the chromatic tetrachord, the term is applied to a succession of notes outside the diatonic scale, and marked by accidentals. A "chromatic scale" is thus a series of semi-tones, and is commonly written with sharps in ascending and flats descending. The most correct method is to write such accidentals as do not involve a change of key.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)