<p><strong>Adverb</strong></p><ul><li>beneath the surface of the ground: traveling underground by subway.</li><li>in concealment or secrecy; not openly: subversion carried on underground.</li></ul><p><strong>Adjective</strong></p><ul><li>existing, situated, operating, or taking place beneath the surface of the ground.</li><li>hidden or secret; not open: underground political activities.</li><li>published or produced by political or social radicals or nonconformists: an underground newspaper.</li><li>avant-garde; experimental: an underground movie.</li><li>critical of or attacking the established society or system: underground opinion.</li><li>of or for nonconformists; unusual: an underground vegetarian restaurant.</li></ul><p><strong>Noun</strong></p><ul><li>the place or region beneath the surface of the ground.</li><li>an underground space or passage.</li><li>a secret organization fighting the established government or occupation forces: He fought in the French underground during the Nazi occupation of France.</li><li>(often initial capital letter) a movement or group existing outside the establishment and usually reflecting unorthodox, avant-garde, or radical views.</li><li>Chiefly British: a subway system.</li></ul><p><strong>Verb</strong></p><ul><li>(used with object) to place beneath the surface of the ground: to underground utility lines.</li></ul>