An orange, also called sweet orange to distinguish it from the bitter orange, is the fruit of a tree in the family Rutaceae. Botanically, this is the hybrid Citrus × sinensis, between the pomelo and the mandarin orange. The chloroplast genome, and therefore the maternal line, is that of pomelo. The sweet orange has had its full genome sequenced.

The juice vesicles, also known as citrus kernels, of a citrus fruit are the membranous content of the fruit's endocarp. The vesicles contain the juice of the fruit and appear shiny and sacklike. Vesicles come in two shapes: the superior and inferior, and these are distinct. Citrus fruit with more vesicles generally weighs more than those with fewer vesicles. Fruits with many segments, such as the grapefruit or pomelo, have more vesicles per segment than fruits with fewer segments, such as the kumquat and mandarin. Each vesicle in a segment in citrus fruits has approximately the same shape, size, and weight. About 5% of the weight of an average orange is made up of the membranes of the juice vesicles.