<p><strong>Adjective</strong></p><ul><li>Born of parents who are not married to each other; born out of wedlock: an illegitimate child.</li><li>Not legitimate; not sanctioned by law or custom.</li><li>Unlawful; illegal: an illegitimate action.</li><li>Irregular; not in good usage.</li><li>Logic. Not in accordance with the principles of valid inference.</li><li>Obsolete. (Formerly, in London) Of or relating to stage plays in which musical numbers were inserted because of laws that gave only a few theaters the exclusive right to produce straight dramas.</li><li>Acting in or producing such productions.</li></ul><p><strong>Noun</strong></p><ul><li>A person recognized or looked upon as illegitimate.</li></ul><p><strong>Verb (used with object)</strong></p><ul><li>Illegitimated, illegitimating. To declare illegitimate.</li></ul>