The Mendocino Celeron, launched 24 August 1998, was the first retail CPU to use on-die L2 cache. Whereas Covington had no secondary cache at all, Mendocino included 128 KB of L2 cache running at full clock rate. The first Mendocino-core Celeron was clocked at a then-modest 300 MHz but offered almost twice the performance of the old cacheless Covington Celeron at the same clock rate. To distinguish it from the older Covington 300 MHz, Intel called the Mendocino core Celeron 300A. Although the other Mendocino Celerons (the 333 MHz part, for example) did not have an A appended, some people call all Mendocino processors Celeron-A regardless of clock rate.
Source [ wikipedia ]