This word in its most general sense means an instrument, or that by which anything is accomplished. In worship it is that by which the mind is fixed on its object. The Yoginī-Tantra says that the Devī should be worshipped either in pratimā (image), maṇḍala ( A figure frequently drawn or made with various colours. The difference between a maṇḍala and a yantra is that the former is used in the case of any Devatā, whereas, a yantra is appropriate to a specific Devatā only.) or yantra.
At a certain stage of spiritual progress the sādhaka is qualified to worship yantra. The siddha-yogi in inward worship (antar-pūjā) commences with the worship of yantra which is the sign (saṃ keta) of brahma-vijñāna as the mantra is the samketa of the Devatā. It is also said that yantra is so called because it subdues (niyantrana) lust, anger, and the other sins of jīva and the sufferings caused thereby.
The yantra is a diagram engraved or drawn on metal, paper, or other substances, which is worshipped in the same manner as an image (pratimā). As different mantras are prescribed for different worships, so are different yantras. The yantras are therefore of various designs, according to the objects of worship. The one in the image is a Gāyatrī yantra belonging to the devi.
In the centre triangle are engraved in the middle the words -
Śrī Śrī Gāyatrī sva-prasāda siddhim kuru (“Śrī Śrī Gāyatrī Devī: grant me success”)
and at each inner corner there are the bījas, Hrīm ̣ and Hraḥ. In the spaces formed by the intersections of the outer ovoid circles is the bīja “Hrīṃ.” The outside circular band contains the bīja “Tha” which indicates “Svaha,” commonly employed to terminate the feminine mantra or vidyā. The eight lotus petals which spring from the band are inscribed with the bīja, “Hrīm ̣ , Īm ̣ , Hraḥ.” The outermost band contains all the matṛkas, or letters of the alphahet, from akāra to kṣa.
The whole is enclosed in the way common to all yantras by a bhūpura, by which as it were, the yantra is enclosed from the outer world. The yantra when inscribed with mantra, serves (so far as these are concerned) the purpose of a mnemonic chart of that mantra appropriate to the particular Devatā whose presence is to be invoked into the yantra.
Certain preliminaries precede, as in the case of a pratimā, the worship of a yantra. The worshipper first meditates upon the Devatā, and then arouses Him or Her in himself. He then communicates the divine presence thus aroused to the yantra. When the Devatā has by the appropriate mantra been invoked into the yantra, the vital airs (prāṇa) of the Devatā are infused therein by the prāṇā-pratiṣṭhā ceremony, mantra, and mudrā. The Devatā is thereby installed in the yantra, which is no longer mere gross matter veiling the spirit which has been always there, but instinct with its aroused presence, which the sādhaka first welcomes and then worships. Mantra in itself is Devatā and yantra is mantra in that it is the body of the Devatā who is mantra.