A measure of a company's total value. It is estimated by determining the cost of buying an entire business in its current state.

Often referred to as "market cap", it is the total dollar value of all outstanding shares. It is calculated by multiplying the number of shares outstanding by the current market price of one share.

Investopedia Says:
Brokerages vary on their exact definitions, but the current approximate classes of market capitalization are:

Mega Cap: Market cap of $200 billion and greater
Big/Large Cap: $10 billion to $200 billion
Mid Cap: $2 billion to $10 billion
Small Cap: $300 million to $2 billion
Micro Cap: $50 million to $300 million
Nano Cap: Under $50 million

If a business has 50 shares, each with a market value of $10, the business's market capitalization is $500 (50 shares x $10/ share).