A type of stock fund that invests in mid-sized companies. A company's size is determined by its market capitalization, with mid-sized firms generally ranging from $2 billion to $10 billion in market cap. Most stocks held in a mid-cap fund are firms with established businesses that are still considered developing companies. These funds tend to offer more growth than large-cap stocks and less volatility than the small-cap segment.The size restrictions for a mid-cap stock fluctuates between funds. The range of $2 billion to $10 billion is only an approximation, and it can change over time.