L1

L1 is an abbreviation used across disciplines to denote either a top-level concept such as Level 1 or Layer 1, or a shorthand used in equations and nomenclature. The meaning is context-dependent and may refer to hardware, mathematics, or linguistics.

In computer architecture, L1 cache is a small, extremely fast memory located on or near each processor core. It typically holds a few kilobytes to tens of kilobytes of data and instructions, with separate L1 instruction and data caches in many CPUs. The cache reduces latency by keeping frequently accessed values close to the execution unit. L1 caches are faster but smaller than L2 and L3 caches, which reside further away or share across cores.

In networking, Layer 1 of the Open Systems Interconnection model is the physical layer, which defines the hardware means of sending raw bit streams—electrical, optical, or radio signals—over a physical medium. It covers cables, connectors, transmitters, receivers, and signaling standards; it does not concern itself with data structure or error detection, tasks handled by higher layers.

In mathematics, the L1 norm of a vector is the sum of the absolute values of its components, often denoted ||x||_1. It induces the L1 distance and is used in optimization problems, including sparse recovery and regularization (L1 regularization).

In linguistics and education, L1 denotes a person’s first language or native language, distinguished from L2.