Ajna is symbolised by a lotus with two petals, and corresponds to the colors violet, indigo or deep blue. It is at this point that the 2 side nadis Ida and Pingala are said to terminate and merge with the central channel Sushumna, signifying the end of duality. The seed syllable for this chakra is the syllable OM, and the presiding deity is Ardhanarishvara, who is a half male, half female Shiva/Shakti. The Shakti goddess of Ajna is called Hakini.
Ajna (along with Bindu), is known as the third eye chakra and is linked to the pineal gland which may inform a model of its envisioning. The pineal gland is a light sensitive gland that produces the hormone melatonin which regulates sleep and waking up. Ajna's key issues involve balancing the higher & lower selves and trusting inner guidance. Ajna's inner aspect relates to the access of intuition. Mentally, Ajna deals with visual consciousness. Emotionally, Ajna deals with clarity on an intuitive level. (Note: some opine that the pineal and pituitary glands should be exchanged in their relationship to the Crown and Brow chakras, based on the description in Arthur Avalon's book on kundalini called Serpent Power or empirical research.)
In Tibetan Buddhism, this point is actually the end of the central channel, since the central channel rises up from the sexual organ to the crown of the head, and then curves over the head and down to the third eye. While the central channel finishes here, the two side channels continue down to the 2 nostrils.
Source - Wikipedia