Trikaya: Three Modalities of Being

Skt., trikaya
three Bodies
Tib., sku-gsum

Trikaya is a concept of three interpenetrating spheres of being, three aspects of the ineffable, three modalities of the divine impulse.
In more practical terms, this can perhaps be understood most readily by using the allegory of ice, water and steam - the three modalities of hydrogen. Ice, the most dense mode of being (for hydrogen), corresponds to the Nirmanakaya, whereas steam (the most subtle modality) corresponds to the Dharmakaya.

Dharmakaya
(Tib., chos-kyi-sku; chos sku, dharma-body, Body of Truth)
The absolute Buddha-nature, the sphere of the essential/absolute mode of being, awareness of emptiness, clear light, space and knowledge. On this level, one finds the primordeal Adibuddha.
Sambhogakaya
(Tib., longs-spyod-rDzogs-sku; longs spyod sku, enjoyment-body)
The sphere of visionary being, the realm of the collective unconscious, the visionary and communicative realm where instructions are being given by visionary appearance of gentle and fierce deities and/or by sound-vibrations. On this level, one finds the five Wisdom Dakinis and the five Dhyani Buddhas
Nirmanakaya
(Tib., sprul-pa'i-sku; sprul sku [pron. tulku], emanation-body, Body of Creation)
The sphere of apparitional being and illusory emanation, the incarnate form that can be perceived by the human senses, earthly manifestations of the Buddha-nature in the form of bodhisattvas and other enlightened beings such as the historical Buddha, Garab Dorje and many others.

Levels 2. and 3. are also indicated by a shared name, rupakaya (Tib., gzugs-kyi-sku, form body), which allows for communication with others, for example humans.

Encompassing and transcending the three levels of the trikaya is the Essential Body of Intrinsic Nature (Skt., Svabhavikakaya)