The word "tantra" means (at least) four different things:
Tantra (1) – A tradition of supernatural ritual magic found within many branches of Hinduism, Jainism, Buddhism, and to a lesser extent Daoism. Also known as Tantrism.
Tantra (2) – A ritual, formula, or procedure described in a text or esoteric teaching, especially in Hinduism and in Vajrayāna (Tibetan Buddhism).
Tantra (3) – A written text describing a tantra (2) and recognized as authentic by one or more sects of one of the Asian religions. Most date from the 5th through 10th centuries CE when religious turmoil and increased literacy led to a great upsurge in interest in learning and recording the secrets of ancient magic.
Tantra (4) – Tantric sex, the common Western term for a set of sexual practices that are allegedly derived from sexual rituals that originated roughly 2500-5000 years ago in the Indus Valley region of India. Also known as neotantra, modern tantra, urban tantra, and sacred sex.