The lemon (Citrus limon) is a small evergreen tree and the tree's yellow fruit. Lemon fruit is used for culinary and nonculinary purposes throughout the world — primarily for its juice, though the pulp and rind (zest) are also used, mainly in cooking and baking. Lemon juice is approximately 5% citric acid, which gives lemons a sour taste. This makes lemon juice an inexpensive acid for use in educational science experiments.

Limonin is a limonoid, and a bitter, white, crystalline substance found in citrus and other plants. It is also known as limonoate D-ring-lactone and limonoic acid di-delta-lactone. Chemically, it is a member of the class of compounds known as furanolactones.

Limonin is enriched in citrus fruits and is often found at higher concentrations in seeds, for example orange and lemon seeds. Limonin is also present in plants such as those of the Dictamnus genus. Limonin and other limonoid compounds contribute to the bitter taste of some citrus food products. Researchers have proposed removal of limonoids from orange juice and other products (known as "debittering") through the use of polymeric films.