Flexibility is defined as:

Flexibility is fundamentally defined as the range of motion available in a joint or group of joints. This capability allows for optimal movement patterns and can lead to improved athletic performance, reduced risk of injuries, and greater overall functional ability.

In the context of fitness, flexibility encompasses the ability to move joints through their full range of motion without stiffness or restriction. A well-developed flexibility level permits smoother and more efficient movements, contributing to activities such as stretching, dancing, and various sports.

The other options pertain to different aspects of physical fitness. Lifting weights relates to strength training, which is not the same as flexibility. Muscle strength refers to the amount of force a muscle can produce, while endurance is associated with the cardiovascular system's capacity to sustain prolonged exercise. These definitions highlight the importance of recognizing that flexibility specifically targets joint mobility, making it distinct from strength and cardiovascular health.