Noun
L1 (plural L1s)
Mother tongue; someone's native language; own language.
Coordinate term: L2
L1
(astrophysics) Abbreviation of Lagrange point 1, located between the larger and smaller objects along a line connecting the two.
(computing) Short for first level in various contexts.
(computing) The first-level cache of a processor, closer to the processor than the second-level cache.
(networking) The physical layer in the seven-layer OSI network model.
Source: en.wiktionary.org