Several centuries after his death, these three facets of the Buddha's nature were articulated in the form of a doctrine developed initially by the Sarvāstivāda school, but quickly taken up and elaborated by the Mahāyāna. According to this development, the Buddha, and all Buddhas were, in their essential nature, identical with the ultimate truth or absolute reality. This is their first ‘body’. At the same time, Buddhas have the power to manifest themselves in a sublime celestial form in splendid paradises where they teach the doctrine surrounded by hosts of bodhisattvas and supernatural beings. This is their second body. Furthermore, motivated by boundless compassion, they project themselves into the world of suffering beings (e.g. the human world) disguised in an appropriate manner through the use of skilful means (upāya-kauśalya) so as not to frighten and alarm, but instead to provide that which is most necessary and useful. This is their third body.
A Buddha in human form is called a Nirmāṇakāya (Skt., ‘transformation body’) and one in celestial form is called a Saṃbhogakāya (Skt., ‘enjoyment body’); the identification of these two bodies with particular figures varies with the lineage (see BUDDHIST SCHOOLS). The unmanifest form is the Dharmakāya (Skt., ‘dharma body’) which is synonymous with Tathatā (Skt., ‘Thus-ness’) and Tathāgatagarbha (Skt., ‘womb or embryo of the Buddhas’). In the Tantric tradition, the Dharmakāya is said to be manifest as an Ādi-Buddha (Skt., ‘Original Buddha’)—identified in different lineages as Vajradhara, Vairocana, Samantabhadra, etc.—who is non-dual with his unmanifest ultimate nature. The unity of the Trikāya is sometimes taught as a fourth aspect, Svabhāvikakāya (Skt., ‘essential body’).
"Trikāya." The Concise Oxford Dictionary of World Religions. . Encyclopedia.com. (July 17, 2018). http://www.encyclopedia.com/religion/dictionaries-thesauruses-pictures-and-press-releases/trikaya
"Trikāya." The Concise Oxford Dictionary of World Religions. . Retrieved July 17, 2018 from Encyclopedia.com: http://www.encyclopedia.com/religion/dictionaries-thesauruses-pictures-and-press-releases/trikaya