Tension is a force along the length of a medium, in particular, a force carried by a flexible medium, such as a rope or cable. The word "tension" comes from Latin, meaning "to stretch". Not coincidentally, the flexible cords that carry muscle forces to other parts of the body are called tendons. Any flexible connector, such as a string, rope, chain, wire, or cable, can exert pull only parallel to its length; so, a force carried by a flexible connector is a tension with a direction parallel to the connector. It is important to understand that tension is a pull in a connector. Consider the saying "you can't push a rope"; tension force only pulls outward along the two ends of a rope.

Flexible connectors are often used to transmit forces around corners, such as in a hospital traction system, a finger joint, or a bicycle brake cable. If there is no friction, the tension is transmitted undiminished. Only its direction changes, and it is always parallel to the flexible connector. For instance, tendons in the finger carry force from the muscles to other parts of the finger, usually changing the force's direction but not its magnitude. The tendons are relatively friction-free. Similarly, a brake cable on a bicycle carries the tension from the handlebars to the brake mechanism. Again, the direction, but not the magnitude, of the tension force is changed.