Vajrayāna (Sanskrit: वज्रयान, lit. 'Diamond Vehicle or Thunderbolt Vehicle'), otherwise known as Mantrayāna ("Mantra Vehicle"), Guhyamantrayāna ("Secret Mantra Vehicle"), Tantrayāna ("Tantra Vehicle"), Indo-Tibetan Buddhism, Tantric Buddhism, and Esoteric Buddhism, is a Mahāyāna Buddhist tradition that emphasizes esoteric practices and rituals aimed at rapid spiritual awakening. Emerging between the 5th and 7th centuries CE in medieval India, Vajrayāna Buddhism incorporates a range of techniques, including the use of mantras (sacred sounds), dhāraṇīs (mnemonic codes), mudrās (symbolic hand gestures), mandalās (spiritual diagrams), and the visualization of deities and Buddhas. These practices are designed to transform ordinary experiences into spiritual paths toward enlightenment and liberation, often by engaging with aspects of desire and aversion in a ritualized context.

A distinctive feature of Vajrayāna Buddhism is its emphasis on esoteric transmission, where teachings are passed directly from teacher (guru or vajrācārya) to student through initiation ceremonies. Tradition asserts that these teachings have been passed down through an unbroken lineage going back to the historical Buddha (c. the 5th century BCE), sometimes via other Buddhas or bodhisattvas (e.g., Vajrapani). This lineage-based transmission ensures the preservation of the purity and effectiveness of the teachings. Practitioners often engage in deity yoga, a meditative practice where one visualizes oneself as a deity embodying enlightened qualities to transform one's perception of reality. The tradition also acknowledges the role of feminine energy, venerating female Buddhas and ḍākiṇīs (spiritual beings), and sometimes incorporates practices that challenge conventional norms to transcend dualistic thinking.

Vajrayāna has given rise to various sub-traditions across Asia. In Tibet, it evolved into Tibetan Buddhism, which became the dominant spiritual tradition, integrating local beliefs and practices. In Japan, it influenced Shingon Buddhism, established by Kūkai, emphasizing the use of mantras and rituals. Chinese Esoteric Buddhism also emerged, blending Vajrayāna practices with existing Chinese Buddhist traditions, however this is no longer a separate sect and has mostly mixed in with normal exoteric Buddhism. Each of these traditions adapted Vajrayāna principles to its cultural context while maintaining core esoteric practices aimed at achieving enlightenment.

Central to Vajrayāna symbolism is the vajra, a ritual implement representing indestructibility and irresistible force, embodying the union of transcendental wisdom and compassion. Practitioners often use the vajra in conjunction with a bell during rituals, symbolizing the integration of male and female principles. The tradition also employs rich visual imagery, including mudrās, complex mandalās, and depictions of wrathful deities that serve as meditation aids to help practitioners internalize spiritual concepts and confront inner obstacles on the path to enlightenment.

In medieval India, the initial terms that were used to refer to the tantric Buddhist tradition were Mantranāya ("Path of Mantras") and Mantrayāna ("Mantra Vehicle"). Later, other terms were adopted, like Vajrayāna ("Diamond Vehicle" or "Thunderbolt Vehicle").

In Tibetan Buddhism practiced in the Himalayan regions of India, Nepal, Tibet, and Bhutan, Buddhist Tantra is most often termed Vajrayāna (Tib. རྡོ་རྗེ་ཐེག་པ་, dorje tekpa, Wyl. rdo rje theg pa) and Secret mantra (Skt. Guhyamantra, Tib. གསང་སྔགས་, sang ngak, Wyl. gsang sngags). The vajra is a mythical weapon associated with Indra that was said to be indestructible and unbreakable (like a diamond) and extremely powerful (like a thunder). Thus, the term is variously translated as Diamond Vehicle, Thunderbolt Vehicle, Indestructible Vehicle, and so on.

The vajra, or diamond, is an adamantine symbol. A diamond has two qualities: it has the power to shatter all ordinary stones, and conversely, no stone can break a diamond. The actual vajra is the mind that has the realization of the ultimate truth inseparably merged with a special kind of tantric bliss. [...] That is why the vajra, or diamond, is the symbol of the tantric path and its result: the wisdom of nondual emptiness and bliss.

In Chinese Esoteric Buddhism, it is generally known by terms such as Zhēnyán (Chinese: 真言, literally "true word", referring to mantra), Tángmì or Hanmì (唐密 - 漢密, "Tang Esotericism" or "Han Esotericism"), Mìzōng (密宗, "Esoteric Sect"), or Mìjiao (Chinese: 密教; Esoteric Teaching). The Chinese term mì 密 ("secret, esoteric") is a translation of the Sanskrit term Guhya ("secret, hidden, profound, abstruse").