Slugs are gastropods without or with very small shells, in contrast with snails, which have a prominent shell. Although they undergo torsion during development, their bodies are streamlined and worm-like, and so show little external evidence of it. This same basic design developed independently in several different groups, the largest being the sea slugs or nudibranchs. Other slugs are found on land, but their soft, slimy bodies are prone to desication, so they are confined to moist environments. Some are notable garden pests.
A slug is also an imperial unit of mass. The term metric slug appears as a footnote in the 1967 seventh ed of Marks Standard Handbook for Mechanical Engineers. It is the mass that accelerates by when a force of one pound (lbf) is exerted on it. On Earth, a mass of 1 slug weighs 32.174 pounds. See the external link for more info.
A slug can also refer to a solid ballistic projectile, often fired from a shotgun. A term derived from this is slug-flow, which is a way in which a gas/liquid mixture can flow through a pipe. The liquid forms a series of slugs occupying the full diameter of the pipe but separated from eachother by large bubbles.
A slightly different from the last definition is the slug that is a bullet that has been reduced in diameter, or has become deformed, by passing from a larger to a smaller part of the bore of the barrel.