Using "margin" means borrowing money from your broker to purchase more stock than you could afford using only your available cash. Think of it like a credit card that you can only use to buy stock. In this case, there can be a tremendous upside to taking the risk of utilizing margin, but there is also a significant downside if the investment goes against you - even more of a downside than just the interest you pay on the borrowed money.The Federal Government (Regulation T) allows you to borrow up to 50% of the initial purchase price of a position, called "initial margin". Beyond that requirement, brokerages require a minimum equity maintenance to be kept to minimize potential losses to you and to them. These minimum maintenance requirements can vary.