Mikkyō (密教?, lit. "secret teachings", "esoteric, Tantric Buddhism") is a Japanese term that refers to the esoteric Vajrayāna practices of the Shingon Buddhist school and the related practices that make up part of the Tendai and Kegon schools.
There are also various Shingon- and Tendai-influenced practices of Shugendō.
Mikkyō is a little-understood, yet often sensationalized, synergistic “esoteric construct” that lies at the very core of Japanese spirituality and mysticism.
Mikkyō is a "lineage tradition": meaning that, as well as instruction in the teachings and practices of the tradition, it also involves and requires “kanjo enablements” - initiatorial empowerment-transmissions - from a master of the Mikkyō disciplines.
The collection of teachings and practices that eventually came to be known as Mikkyō had its early beginnings in the esoteric traditions of India and China.
As early as the 6th Century, there had begun a major importation of spiritual and cultural ideas into Japan from China.
However, it was in the early 9th Century that the formative concepts which would in time become the core of “mainstream” Mikkyo - Shingon and Tendai - were brought to Japan - initially by the monks Kūkai (the founder of Shingon) and Saichō (the founder of Tendai), both of whom had traveled to China to study.
To these initial doctrines & beliefs were later added teachings concerning the powers of mysticism, magic and healing that had gradually begun to reach Japan with the arrival of itinerant monks, priests, hermits and shamanic practitioners, forced for various reasons to flee from China after the fall of the Tang Dynasty.
Blending easily with elements of Shinto practice and the pre-Buddhist folk traditions of sangaku-shinkō - “spiritual practices connected with sacred mountains”, these imported teachings, combining Chinese Tantric Buddhism, Chinese Yin-Yang magic, Taoism and, at a later date, Tibetan Vajrayāna Buddhism, evolved to become the esoteric Japanese tradition that is Mikkyō.