A machine is any mechanical or organic device that transmits or modifies energy to perform or assist in the performance of tasks. It normally requires some energy source ("input") and accomplishes some sort of work.

People have used mechanisms and machines to amplify their abilities since before written records were available. Generally these devices decrease the amount of force required to do a given amount of work, alter the direction of the force, or transform one form of motion or energy into another.

The mechanical advantage of a simple machine is the ratio between the force it exerts on the load, and the input force applied. This does not entirely describe the machine's performance, as force is required to overcome friction, as well. The mechanical efficiency of a machine is the ratio of the actual mechanical advantage (AMA) to the ideal mechanical advantage (IMA). Functioning physical machines are always less than 100% efficient.

Modern power tools, automated machine tools, and human-operated power machinery complicate the definition of "machine" greatly. Machines used to transform heat or other energy into mechanical energy are known as engine.