Short form for Level 1 Cache, AKA Primary Cache. The L1 Cache puts data in a small amount of SRAM, speeding up system performance. It is usually integrated into the CPU chip, storing instructions as they are executed, and is not usually very large (typically 16K). It is located between the CPU registers and the L2 Cache. It's useful for storing some instructions, but usually relies on an L2 Cache for an all-around performance boost.
info from http://www.cnet.com/Resources/Info/Glossary/Terms/primarycache.html and http://www.geek.com/glossary/glossary_search.cgi?l1 and http://www.amd.com/us-en/Glossary/0,,238_242~1042,00.html