Tushita or Tusita is one of the six deva-worlds of the Kamadhatu, located between the Yama heaven and the Nirmanarati heaven. Like the other heavens, Tusita is said to be reachable through meditation. It is the heaven where the Bodhisattva Svetaketu (Pali: Setaketu "White Banner") resided before being reborn on Earth as Gautama Buddha, the historical Buddha; it is, likewise, the heaven where the bodhisattva Natha ("Protector") currently resides, who will later be born as the next Buddha, Maitreya.
Like all heaven realms in Buddhism, the Tusita Heaven is the residence of divine beings or devas. According to the Visakhuposatha Sutta of the Pāli Canon, time runs very differently from on Earth:
That which among men is four hundred years, Visakha, is one night and day of the Tusita devas, their month has thirty of those days, their year twelve of those months; the lifespan of the Tusita devas is four thousand of those heavenly years...
In Mahayana Buddhist thought, the Tusita Heaven is where all Bodhisattvas destined to reach full enlightenment in their next life dwell for a time. One such reference can be found in the Infinite Life Sutra, a Mahayana text:
Each of these bodhisattvas, following the virtues of the Mahasattva Samantabhadra, is endowed with the immeasurable practices and vows of the Bodhisattva Path, and firmly dwells in all the meritorious deeds. He freely travels in all the ten quarters and employs skillful means of emancipation. He enters the treasury of the Dharma of the Buddhas, and reaches the Other Shore. Throughout the innumerable worlds he attains Enlightenment. First, dwelling in the Tusita Heaven, he proclaims the true Dharma. Having left the heavenly palace, he descends into his mother's womb.
The Tuṣita heaven is therefore closely associated with Maitreya, and many Buddhists vow to be reborn there so that they can hear the teachings of the Bodhisattva and ultimately be reborn with him when he becomes a Buddha. Other bodhisattvas dwell in this heaven realm from time to time.