Śūnyatā (शून्यता) is the Sanskrit term rendered in English as emptiness. In Chinese kōng (空), in Japanese kū, in Tibetan stong pa nyid.
To avoid splitting the teaching across two pages, the full treatment — the doctrine, Nāgārjuna‘s argument, the two truths, readings across Mahayana, Theravāda, Zen, and Tibetan Buddhism, and careful distinctions from Taoist wu, apophatic theology, and Advaita’s Brahman — is at Emptiness.
The Heart Sūtra‘s most-quoted line states it as compactly as any:
rūpaṃ śūnyatā śūnyataiva rūpaṃ Form is emptiness; emptiness is form.