The meanings of "big cap" and "small cap" are generally understood by their names: big-cap stocks are shares of larger companies and small-cap stocks are shares of smaller companies. Labels like these, however, are often misleading. If you don't realize how big "small-cap" stocks have become, you'll miss some good investment opportunities.
Small-cap stocks are often considered good investments due to their low valuations and potential to grow into big-cap stocks, but the definition of small-cap has changed over time. What was considered a big-cap stock in 1980 is a small-cap stock today. This article will define the "caps" and provide additional information that will help investors understand terms that are often taken for granted.
First, we need to define "cap," which refers to market capitalization and is calculated by multiplying the price of a stock by the number of shares outstanding. Generally speaking, this represents the market's estimate of the "value" of the company; however, it should be noted that while this is the common conception of market capitalization, to calculate the total market value of a company, you actually need to add the market value of any of the company's publicly traded bonds.
Source : Investopedia