Free body is the generic term used by physicists

Physics

Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

 and engineers

Engineering

Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...

 to describe an object—be it a bowling ball

Bowling ball

A bowling ball is a spherical ball made from plastic, reactive resin, urethane or a combination of these materials which is used in the sport of bowling. Ten-pin bowling balls generally have a set of three holes drilled in them, one each for the ring and middle finger, and one for the thumb;...

, a spacecraft

Spacecraft

A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

, pendulum

Pendulum

A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

, a television

Television

Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...

, or anything else—which can be considered as moving as a single unit. The object doesn't have to be "free" in the usual sense of the word—it could be completely prevented from going anywhere, or it could be trapped in an orbit

Orbit

In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

. To study the stresses acting in a material, small elements or sections of beams

Beam (structure)

A beam is a horizontal structural element that is capable of withstanding load primarily by resisting bending. The bending force induced into the material of the beam as a result of the external loads, own weight, span and external reactions to these loads is called a bending moment.- Overview...

 are treated as free bodies. The crucial concept is that the physicist can think of it as a single unit to the extent that it either does or does not move, as the case may be.
Often the term is associated with a free body diagram

Free body diagram

A free body diagram, also called a force diagram, is a pictorial representation often used by physicists and engineers to analyze the forces acting on a body of interest. A free body diagram shows all forces of all types acting on this body. Drawing such a diagram can aid in solving for the unknown...

.