The ushnisha (Sanskrit, n., उष्णीष, IAST: uṣṇīṣa) is a three-dimensional oval at the top of the head of the Buddha.
It symbolizes his attainment of reliance in the spiritual guide.
The ushnisha was not described initially in the Physical characteristics of the Buddha spelled out by the Buddhist canon. Rather, there are several mentions about a topknot:
"His topknot is like a crown." (Secondary characteristics, No53)
"He has a topknot as if crowned with a flower garland." (Secondary characteristics, No80)
The first representations of the Buddha in the 1st century CE in the Greco-Buddhist art of Gandhara also represent him with a topknot, rather than just a cranial knob.
It is thought that the interpretation of the ushnisha as a supernatural cranial protuberance happened at a later date, as the representation of the topknot became more symbolized and its original meaning was lost (Mario Bussagli, "L'art du Gandhara").
The Boddhisattva-Cakravartin in Early Buddhism
A Sikh boy wearing a patka, the headgear called uṣṇīṣa in Early Buddhism
In Early Buddhism, the uṣṇīṣa was represented differently.
The Mahāvastu (1.259f) and the Divyāvadāna, as well as the Theravadin Milindapañha, describe the marks of the cakravartin or "idealised world-ruler":
uṣṇīṣa or patka turban, chhatra "parasol", "horn jewel" or vajra, whisk and sandals.
These were the marks of the kshatriya.
The plastic art of early Mahayana Buddhism in Mathura presents bodhisattvas in a form called uṣṇīṣin "wearing a turban/hair binding", wielding the mudras for "nonviolent cakravartin rule".