Debit Balance

Buying a position on margin entails borrowing against a portion of the value of other securities in an account in order to increase the size of a position.

You own 1,000 shares of Whatever.com, which trades for $40 a share. That's all you have in your account: 40 grand. If you had a 50% margin limit, you could borrow up to $20,000 on that account, presumably to invest it or buy a car or throw a great Bar Mitzvah. The total account exposure then would have $60,000. Should things go awry, however, and that $40 stock trades down to $30, then the account holder is carrying a debit balance. The kindly broker will be forced to sell, whether you like it or not, $10,000 worth of stock, generating that much cash and taking the margin balance below the 50% max threshold.

Bottom line: You don't want this.