Manipura, the city of jewels, is described in the Sat-Cakra-Nirupana as a lotus of ten petals at the navel, bearing the letters dam through pham. Within it is a red triangle with swastika marks at its sides, the region of fire, with the seed syllable RAM seated on a ram; the presiding deity is Rudra with the shakti Lakini. The association of the navel region with fire connects to a much wider Indian medical and yogic idea of the digestive fire, agni or jatharagni, which is central to Ayurveda, so this is one of the centres where the tantric scheme meets an older and independent physiological doctrine. The correspondence with the solar plexus is a modern anatomical gloss, not a term the Sanskrit sources use.
The idea that the navel region houses a fire with wider significance than digestion recurs outside the Hindu tantric tradition as well: in Tibetan Vajrayana Buddhism, the practice known as tummo, inner heat, one of the Six Yogas of Naropa, is centred on the navel and is used to generate physical warmth and, in the tradition's own account, to support more advanced stages of meditation, an independent development rather than a borrowing from the Sanskrit Sat-Cakra-Nirupana but a recognisable case of the same body region being read, in two separate tantric traditions, as a seat of heat and transformative power.
Sources The Serpent PowerSir John Woodroffe (Arthur Avalon) with The Origins of Yoga and Tantra: Indic Religions to the Thirteenth CenturyGeoffrey Samuel