Word/Week
To further define the definition, we have a couple of other perhaps unfamiliar words to examine. Remember that accidentals are the flats, sharps, or naturals that alter a given key signature. When you see many of these symbols used on several notes in a row, that phrase of music might be said to be "chromatic" in nature, hence merriam-webster's 3.b. frequent use of accidentals.
On a piano, to play chromatically, all the keys next to each other would be utilized. Playing a chromatic scale starting with middle C would be played with the following white/black key order. WBWBWWBWBWBWW. Next time you encounter a piano or other musical keyboard try that pattern out to hear how a chromatic scale sounds.