Ajna, meaning command, is described in the Sat-Cakra-Nirupana as a lotus of two petals between the eyebrows, bearing the letters ham and ksham. It is the seat of manas, the mind, and of the shakti Hakini, and within it is the itara linga and the syllable OM; unlike the lower five it is not assigned one of the gross elements, standing above them. The text says that the yogi who concentrates here becomes able to enter another body at will and becomes the knower of all, and that it is here that the guru's command is received, which is the sense of the name. Its position in the scheme, above the elemental centres and below the thousand-petalled lotus, makes it the last of the six centres proper.
Ajna carries a specific name in Hatha yoga literature for the point where the three principal nadis are said to meet: muktatriveni, the liberated triple confluence, comparing the meeting of Ida, Pingala and Sushumna at the brow to the geographic confluence of the Ganga, Yamuna and the legendary underground Saraswati at Prayag, a site of pilgrimage now within the modern city of Prayagraj. From this meeting point the texts describe Ida and Pingala as ending at the two nostrils while Sushumna alone continues upward, unaccompanied, through Ajna to Sahasrara, part of why Ajna is treated as the last of the six elemental centres rather than as a seventh alongside them.
Sources The Serpent PowerSir John Woodroffe (Arthur Avalon) with Roots of YogaJames Mallinson and Mark Singleton (trans. and eds.)