Here's my definition:
A stepping is a "geek" term for revision. As the chip progresses newer revisions/stepping are developped. Newer steppings will remove bugs and so on from previous steppings and can result in a higher overclock.
I also found this on our Overclockers.com website:
What does stepping mean?
Celerons come in four flavors. The C266 and C300 without L2 cache and the C300A and C333 with 128 Kb L2 cache. Each type of Celeron has several slightly different variations, called a "stepping". Stepping 0 (zero) cores are the original production run. When minor imperfections (bugs) are found in the instruction programming (micro-code) of the core or in other parameters of the chip, they are fixed and the next batch of cores will incorporate the changes. This batch will be identified as stepping 1. If another change is required later, the stepping number will be incremented again. As each successive refinement to the chip is made, the next higher stepping number will be assigned. For many reasons, one stepping may be easier to overclock than another, but usually the higher stepping cores make the best, most stable CPU's.