The Collaborative International Dictionary of English v.0.48:

Tantra \Tan"tra\ (t[a^]n"tr[.a]; t[u^]n"tr[.a]), n. [Skr.]
   (Hinduism)
   A ceremonial treatise related to Puranic and magic
   literature; esp., one of the sacred works of the worshipers
   of Sakti. -- Tan"tric (t[a^]n"tr[i^]k), a.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

tantra
    n 1: any of a fairly recent class of Hindu or Buddhist religious
         literature concerned with ritual acts of body and speech
         and mind
    2: doctrine of enlightenment as the realization of the oneness
       of one's self and the visible world; combines elements of
       Hinduism and paganism including magical and mystical elements
       like mantras and mudras and erotic rites; especially
       influential in Tibet [syn: Tantra, Tantrism]