The slug is a unit of mass associated with Imperial units. It is a mass that accelerates by 1 ft/s² when a force of one pound-force (lb_F) is exerted on it.

Failed to parse (Missing texvc executable; please see math/README to configure.): 1\,\text{slug} =1\,\frac{\text{lb}_F\cdot\text{s}^2}{\text{ft}} \qquad\Longleftrightarrow\qquad 1\,\text{lb}_F = 1\,\frac{\text{slug}\cdot\text{ft}}{\text{s}^2}

With standard gravity g_c = 9.80665 m/s², the international foot of 0.3048 m and the avoirdupois pound of 0.45359237 kg, one slug therefore has a mass of approximately 32.17405 lb_m or 14.593903 kg. At the surface of the Earth, an object with a mass of 1 slug exerts a force of about 32.17 lb_F or 143 N.

The slug is part of a subset of units known as the gravitational FPS system, one of several such specialized systems of mechanical units developed in the late 19th and the 20th century. Geepound was another name for this unit in early literature.

The name "slug", as a unit of inertia, was coined before 1900 by British physicist Arthur Mason Worthington, but it did not see any significant use until decades later. A 1928 textbook says:

Slug is a common name that is normally applied to any gastropod mollusc that lacks a shell, that has a very reduced shell, or has a small internal shell. (This is in contrast to the common name snail, applied to gastropods that have a coiled shell large enough that the soft parts of the animal can retract fully into it.)

Slugs belong to several different lineages which also include snails that have shells. The various families of land slugs are not very closely related, despite a superficial similarity in the overall body form. The shell-less condition has arisen many times independently during the evolutionary past, and thus the category "slug" is emphatically a polyphyletic one.

As well as land slugs, there are also many marine slugs and even one freshwater slug genus (Acochlidium), but the common name "slug" is most frequently applied to air-breathing land slugs, while the marine forms are usually known as sea slugs. Land gastropods with a shell that is not quite vestigial, but is too small to retract into (like many in the family Urocyclidae), are known as semislugs.

In physics, mass (from Greek μᾶζα "barley cake, lump (of dough)"), more specifically inertial mass, can be defined as a quantitative measure of an object's resistance to the change of its speed. In addition to this, gravitational mass can be described as a measure of magnitude of the gravitational force which is

when interacting with a second object. The SI unit of mass is the kilogram (kg).

In everyday usage, mass is often referred to as weight, the units of which are often taken to be kilograms (for instance, a person may state that their weight is 75 kg). In scientific use, however, the term weight refers to a different, yet related, property of matter. Weight is the gravitational force acting on a given body—which differs depending on the gravitational pull of the opposing body (e.g. a person's weight on Earth vs on the Moon) — while mass is an intrinsic property of that body that never changes. In other words, an object's weight depends on its environment, while its mass does not. On the surface of the Earth, an object with a mass of 50 kilograms weighs 491 Newtons; on the surface of the Moon, the same object still has a mass of 50 kilograms but weighs only 81.5 Newtons. Restated in mathematical terms, on the surface of the Earth, the weight W of an object is related to its mass m by W = mg, where g is the Earth's gravitational field strength, equal to about 9.81 m/s².