The word fennel developed from the Middle English fenel or fenyl. This came from the Old English fenol or finol, which in turn came from the Latin feniculum or foeniculum, the diminutive of fenum or faenum, meaning "hay". The Latin word for the plant was ferula, which is now used as the genus name of a related plant. As Old English finule it is one of the nine plants invoked in the pagan Anglo-Saxon Nine Herbs Charm, recorded in the 10th century. In Ancient Greek, fennel was called marathon, and is attested in Linear B tablets as ma-ra-tu-wo. John Chadwick noted this word is the origin of the place name Marathon (meaning "place of fennel"), site of the Battle of Marathon in 490 BC. In Greek mythology, Prometheus used the stalk of a fennel plant to steal fire from the gods. Also, it was from the giant fennel, Ferula communis, that the Bacchanalian wands of the god Dionysus and his followers were said to have come.

Fennel, Foeniculum vulgare, is a perennial herb. It is erect, glaucous green, and grows to heights of up to 2.5 m, with hollow stems. The leaves grow up to 40 cm long; they are finely dissected, with the ultimate segments filiform (threadlike), about 0.5 mm wide. (Its leaves are similar to those of dill, but thinner.) The flowers are produced in terminal compound umbels 5–15 cm wide, each umbel section having 20–50 tiny yellow flowers on short pedicels. The fruit is a dry seed from 4–10 mm long, half as wide or less, and grooved.

Fennel is widely cultivated, both in its native range and elsewhere, for its edible, strongly flavoured leaves and fruits, which are often mistermed "seeds". Its aniseed flavour comes from anethole, an aromatic compound also found in anise and star anise, and its taste and aroma are similar to theirs, though usually not as strong.

The Florence fennel (Foeniculum vulgare Azoricum Group; syn. F. vulgare var. azoricum) is a cultivar group with inflated leaf bases which form a bulb-like structure. It is of cultivated origin, and has a mild anise-like flavour, but is more aromatic and sweeter. Florence fennel plants are smaller than the wild type. Their inflated leaf bases are eaten as a vegetable, both raw and cooked. There are several cultivars of Florence fennel, which is also known by several other names, notably the Italian name finocchio. In North American supermarkets, it is often mislabelled as "anise".

Fennel seeds are sometimes confused with those of anise, which are similar in taste and appearance, though smaller. Fennel is also used as a flavouring in some natural toothpastes.

Fennel features prominently in Mediterranean cuisine, where bulbs and fronds are used, both raw and cooked, in side dishes, salads, pastas, vegetable dishes and risottos. Fennel seed is a common ingredient in Italian sausages and meatballs and northern European rye breads[citation needed].

Many cultures in the Indian subcontinent and the Middle East use fennel seed in their cookery. Fennel is one of the most important spices in Kashmiri Pandit and Gujarati cooking.[citation needed] It is an essential ingredient of the Assamese/Bengali/Oriya spice mixture panch phoron and in Chinese five-spice powders. In many parts of Pakistan and India, roasted fennel seeds are consumed as mukhwas, an after-meal digestive and breath freshener. In Bengal fennel is eaten after a meal and sometimes it is accompanied by roasted coriander and ajwain. Fennel leaves are used as leafy green vegetables either by themselves or mixed with other vegetables, cooked to be served and consumed as part of a meal, in some parts of India. In Lebanon, it is used to make a special kind of egg omelette (along with onions, and flour) called ijjeh.

Many egg, fish, and other dishes employ fresh or dried fennel leaves. Florence fennel is a key ingredient in some Italian and German salads, often tossed with chicory and avocado, or it can be braised and served as a warm side dish. It may be blanched or marinated, or cooked in risotto.

Syrup prepared from fennel juice was formerly given for chronic coughs. It is one of the plants which is said to be disliked by fleas, and powdered fennel has the effect of driving away fleas from kennels and stables.