The Heart Sūtra is a popular sutra in Mahāyāna Buddhism. Its Sanskrit title, Prajñāpāramitāhṛdaya, can be translated as "The Heart of the Perfection of Wisdom".
The Sutra famously states, "Form is empty, emptiness is form." (śūnyatā). It is a condensed exposé on the Buddhist Mahayana teaching of the Two Truths doctrine, which says that ultimately all phenomena are sunyata, empty of an unchanging essence. This emptiness is a 'characteristic' of all phenomena, and not a transcendent reality, but also "empty" of an essence of its own. It has been called "the most frequently used and recited text in the entire Mahayana Buddhist tradition."