Incognito

Webster's Dictionary of the English Language

·adj One unknown or in disguise, or under an assumed character or name.

II. Incognito ·adj The assumption of disguise or of a feigned character; the state of being in disguise or not recognized.

III. Incognito (·adj / ·adv) Without being known; in disguise; in an assumed character, or under an assumed title;

— said ·esp. of great personages who sometimes adopt a disguise or an assumed character in order to avoid notice.