Traidhātuka / Three Realms of Existence
In Buddhism, the universe of sentient beings is composed of three realms: (1) the realm of desire (kāmadhātu), where the mind is motivated by sensual pleasures; (2) the realm of subtle forms (rūpadhātu), where the mind becomes uninterested in sensual objects but still engaged in conceptual mental activities; and (3) the realm of the formless (ārūpyadhātu), the higher level of a mind free from desire and conceptualization. Sentient beings are all subject to past and future deeds (karma), therefore trapped in the cycle of rebirth within the three realms. It is thus called the wheel of sufferings in the three realms. The idea of the three realms of existence reveals how the Buddhists understand the universe on the one hand, and how it is in relevance to the different stages of meditative achievements on the other.
Everything in the three realms is like a dream. I shall be motivated with diligence. Having become fully enlightened, I must expound the doctrine to all beings of the three realms. (The Effulgent Perfection of Wisdom Sutra)