कोई भी युक्ति जो उर्जा लेकर कुछ कार्यकलाप करती है उसे यंत्र या मशीन (machine) कहते हैं। सरल मशीन वह युक्ति है जो लगाये जाने वाले बल का परिमाण या दिशा को बदल दे किन्तु स्वयं कोई उर्जा खपत न करे।

A machine is a physical system that uses power to apply forces and control movement to perform an action. The term is commonly applied to artificial devices, such as those employing engines or motors, but also to natural biological macromolecules, such as molecular machines. Machines can be driven by animals and people, by natural forces such as wind and water, and by chemical, thermal, or electrical power, and include a system of mechanisms that shape the actuator input to achieve a specific application of output forces and movement. They can also include computers and sensors that monitor performance and plan movement, often called mechanical systems.