A fly on clothing is a covering over an opening concealing the mechanism, such as a zip, velcro or buttons, used to close the opening. The term is most frequently applied to a short opening over the groin in trousers, shorts and other garments, which makes them easier to put on or take off and allows men to urinate without lowering the garment. The term is also used of overcoats, where a design of the same shape is used to hide a row of buttons. This style is common on a wide range of coats, from single-breasted Chesterfields to covert coats.

An open fly is a fly that has been left unzipped or unbuttoned, usually unintentionally. It is often considered humorous if a person is caught with his or her fly open.^{[citation needed]}

Trousers have varied historically in whether or not they have flies. Originally, trousers did not have flies or other openings, being pulled down for sanitary functions. The use of a codpiece, a separate covering attached to the trousers, became popular in 16th century Europe, eventually evolving into an attached fall-front (or broad fall). The fly-front (split fall) emerged later. The panelled front returned as a sporting option, such as in riding breeches, but is now hardly used, flies being by far the most common fastening. Most flies now use a zip, though button flies continue in use.

True flies are insects of the order Diptera (from the Greek di = two, and ptera = wings). They possess a pair of wings on the mesothorax and a pair of halteres, derived from the hind wings, on the metathorax. Apart from secondarily flightless insects (including some flies), the only other order of insects with any form of halteres are the Strepsiptera, and theirs are on the mesothorax, with the flight wings on the metathorax.

The presence of a single pair of patent, metathoracic flight wings distinguishes true flies from other insects with "fly" in their names, such as mayflies, dragonflies, damselflies, stoneflies, whiteflies, fireflies, alderflies, dobsonflies, snakeflies, sawflies, caddisflies, butterflies or scorpionflies. Some true flies have become secondarily wingless, especially in the superfamily Hippoboscoidea, or among those that are inquilines in social insect colonies.

Some authors draw a distinction in writing the common names of insects. True flies are written as two words, such as crane fly, robber fly, bee fly, moth fly, and fruit fly. In contrast, common names of nondipteran insects that have "fly" in their names are written as one word, e.g., butterfly, stonefly, dragonfly, scorpionfly, sawfly, caddisfly, whitefly. In practice, however, this is a comparatively new convention, and, especially in older books, one commonly might see the likes of "saw fly" and "caddis fly". In any case, non-entomologists cannot in general be expected to tell dipterans, "true flies", from other insects.