Companies having a market capitalization between $2 billion and $10 billion. Shortened form of "middle cap".

Investopedia Says:
As the name implies, mid-cap companies are in the middle of the pack. Mid caps aren't too big, but they have a respectably sized market cap.

Keep in mind that classifications such as "large cap" or "small cap" are only approximations that change over time. Also, the exact definition of these terms can vary between brokerage houses.

Related Links:
Find out the difference between mega-, large-, mid- and small-cap stocks. We show how each suits particular investing styles. Market Capitalization Defined
Learn how to narrow down your list from thousands of choices. Picking The Right Mutual Fund
Different funds will invest in companies with different market caps - find out which is right for you. Determining What Market Cap Suits Your Style
Wall Street is myopically focused on a minority of large-cap stocks, leaving the majority of stocks under-followed and undervalued. The Great Gap

Stock with a middle-level capitalization (numbers of shares outstanding times the price of the shares). Mid Cap stocks typically have between $1 billion and $5 billion in outstanding market value. Many mutual funds specializing in mid cap stocks will use the words mid cap in their names.

Stock with a middle-level Market Capitalization. Mid-cap stocks typically have between $1 billion and $5 billion in outstanding market value. Many mutual funds specializing in mid-cap stocks will use the word ‘Mid-cap' in their names.