The Nasdaq Composite has dropped roughly 13% in less than a month (as of this writing). As distressing as it feels, long-term investors must remember that 10% "corrections" in the market are surprisingly common -- typically occurring once every two years.

Heard of a stock-market correction, but not quite sure what it means? We’ll help you understand what investors are talking about.

The Nasdaq Composite (NASDAQINDEX: ^IXIC) has fallen more than 10% from a recent high. A bear market -- a 20% drop from recent highs -- is not guaranteed to happen, but this is why investors get so worried about a 10% market decline.