Translational motion refers to a type of motion in which a body or object moves along a linear axis rather than a rotational axis. In physics, this term is used when referring to motion that changes the position of an object without rotation, which often plays a key part in molecular behavior. In avionics, it describes the motion of an aircraft along the X, Y, or Z axes — in other words, moving left or right, forward or back, and up or down along lines referred to as vectors. Translational motion is also referred to as linear motion.

The study of translational motion is referred to as translational dynamics, and utilizes a series of equations to measure the motion of objects, as well as the effects of various forces on those objects. These equations use mass, velocity vectors, linear acceleration vectors, linear momentum vectors, and force to determine an object's translational motion. Some forces that act on a body in this type of motion can include gravity and friction.

On the molecular level, translational motion is used to describe the movements of atoms and molecules in a substance, giving the substance its temperature. This is also referred to as the kinetic energy of a substance. Catalysts that change the speed of the particles can raise or lower the temperature of the substance. Such catalysts can include burning or freezing, and can often affect the state of a substance.

For example, heating water increases the speed of the water molecules' translational motion, causing the temperature of the water to rise. When water reaches its vaporization point, it becomes steam, its molecules moving too quickly to retain its liquid form. The speed of translational motion causes the liquid to become a gas, dispersing the molecules over a wider area. Conversely, freezing water causes the speed of the molecules to slow. The water becomes ice, a dense solid in which the water molecules move too slowly to disperse.

Most motion in the physical world is a combination of translational motion and rotational motion, in which rotational motion controls the direction on the axis while translational motion propels the object in that direction. The human body moves with a combination of these two types of motion. The limbs rotate on their joints, providing the impetus for directional motion, such as walking.

Experiments have determined that combined translational motion and rotational motion is more efficient in terms of kinetic energy than translational alone. Pure translational motion creates constant friction against its surrounding surfaces, even the air, causing greater loss of kinetic energy and momentum over time. Adding rotational motion reduces the friction, allowing kinetic energy to persist for a longer period.