The bael fruit is the fruit of the bael tree, a subtropical species of tree that grows wild in dry, forested areas of the Indian subcontinent, and is mixed deciduous areas of Southeast Asia.
The fruit produced by the tree is described this way in the book Fruits of Warm Climates, written by Julia F. Morton and published in 1987:
The fruit, round, pyriform, oval, or oblong, 2 to 8 in (5-20 cm) in diameter, may have a thin, hard, woody shell or a more or less soft rind, gray-green until the fruit is fully ripe, when it turns yellowish. It is dotted with aromatic, minute oil glands. Inside, there is a hard central core and 8 to 20 faintly defined triangular segments, with thin, dark-orange walls, filled with aromatic, pale-orange, pasty, sweet, resinous, more or less astringent, pulp. Embedded in the pulp are 10 to 15 seeds, flattened-oblong, about 3/8 in (1 cm) long, bearing woolly hairs and each enclosed in a sac of adhesive, transparent mucilage that solidifies on drying.
The bael tree is cultivated in many of the same areas where it grows wild, and the fruit itself is consumed for its perceived medicinal benefits in addition to being enjoyed by many as a food. According to one source, "the skin of some forms of the fruit is so hard it must be cracked open with a hammer."
Bael fruit can be eaten fresh or dried, and can also be juiced. The bael fruit is also a very nutritious food, and can be used as an ingredient in candies and nectars. Dried bael fruit slices are also used to make tea.
Another book that described the bael fruit is the Encyclopedia Brittanica:
BAEL FRUIT (Aegle marmelos). Aegle is a genus of the botanical natural order Rutaceae, containing two species in tropical Asia and one in west tropical Africa. The plants are trees bearing strong spines, with alternate, compound leaves each with three leaflets and panicles of sweet-scented white flowers. Aegle marmelos, the baelor bel-fruit tree (also known as Bengal quince), is found wild or cultivated throughout India. The tree is valued for its fruit, which is oblong to pyriform in shape, 2-5 in. in diameter, and has a grey or yellow rind and a sweet, thick orange-coloured pulp. The unripe fruit is cut up in slices, sun-dried and used as an astringent; the ripe fruit is described as sweet, aromatic and cooling. The wood is yellowish-white, and hard but not durable. The name Aegle is from one of the Hesperides, in reference to the golden fruit; marmelos is Portuguese for quince.