In Hinduism and Buddhism, the Sanskrit term Bīja(बीज) (Jp. 種子 shuji) (Chinese 种子 zhǒng zǐ), literally seed, is used as a metaphor for the origin or cause of things and cognate with bindu.
The metaphor is considerably extended in the Consciousness-only teachings of the Yogacara school of Buddhism. According to this theory, all experiences and actions produce bīja as impressions, stored in the alaya (storehouse) consciousness. The external world is produced when the seeds "perfume" this consciousness. This view of bīja has been equated to memes, with the theory itself positing an extreme form of memetics (i.e. reality and existence consist purely of memes).
In Vajrayana Buddhism and Hinduism, the term bīja is used for mystical "seed syllables" contained within mantras. These seeds do not have precise meanings, but are thought to carry connections to spiritual principles. The best-known bīja syllable is Om, first found in the Hindu scriptures the Upanishads.
Khanna (2003: p. 21) links mantras and yantras to thoughtforms:
Mantras, the Sanskrit syllables inscribed on yantras, are essentially 'thought forms' representing divinities or cosmic powers, which exert their influence by means of sound-vibrations.
In some tantric traditions, the Bija of the 'Varnamala' (Sanskrit; English: "garland of letters"; which may be rendered as alphabet) are understood as aniconic representations and sound embodiments of the matrikas (a group of goddesses).
In Tibetan Buddhism the seed syllables corresponding to the Three Vajras are: a white oṃ (enlightened body), a red āḥ (enlightened speech) and a blue hūṃ (enlightened mind).
In the Bön tradition of Tibet, it's a little different: a white āḥ, a red oṃ and a blue hūṃ.
Bijas are often the vehicle of esoteric transmission of terma to a 'tertön' (Tibetan; English: "revealer of terma"), such as that experienced by Dudjom Lingpa.