L5

L5 may refer to several distinct concepts across different fields. In human anatomy, L5 denotes the fifth lumbar vertebra, located in the lower back. It bears a substantial portion of the body's weight and forms joints with L4 above and S1 below. The L5 vertebra is a common site of degenerative changes and herniation that can contribute to lower back pain and radicular symptoms.

In information technology, Layer 5 of the OSI model is the Session layer. This layer is responsible for managing and controlling dialog between applications, establishing, maintaining, and terminating communication sessions, and providing synchronization and checkpointing as needed. In practice, many networks implement these functions in higher layers or at endpoints rather than as a distinct protocol.

In astronomy and astrodynamics, L5 refers to the fifth Lagrange point in a two-body system. The L4 and L5 points lie at the third vertex of an equilateral triangle formed with the two primary bodies and are points of gravitational equilibrium. They can host stable natural bodies, such as Trojan asteroids, or be used for spacecraft that share an orbit with a planet.

The designation L5 is also used as a generic label in various product names, research projects, and model numbers, where the exact meaning depends on the organization and domain.