What does accessory mean?
The definition of accessory
Ac*ces”so*ry (#; 277), a.
Etym: [L. accessorius. See Access, and cf. Accessary.]

Accompanying as a subordinate; aiding in a secondary way; additional; connected as an incident or subordinate to a principal; contributing or contributory; said of persons and things, and, when of persons, usually in a bad sense; as, he was accessory to the riot; accessory sounds in music.

Note: Ash accents the antepenult; and this is not only more regular, but preferable, on account of easiness of pronunciation. Most orhoëpists place the accent on the first syllable.
Synonyms
— Accompanying; contributory; auxiliary; subsidiary; subservient; additional; acceding.

Ac*ces”so*ry, n.; pl. Accessories.
1 That which belongs to something else deemed the principal; something additional and subordinate. “The aspect and accessories of a den of banditti.” Carlyle.
2 (Law)
Same as Accessary, n.
3 (Fine Arts)
Anything that enters into a work of art without being indispensably necessary, as mere ornamental parts. Elmes.

Synonyms
— Abettor; accomplice; ally; coadjutor. See Abettor.