The amount remaining in a cash account or margin account after all securities have been paid for.
In a cash account, the credit balance is the amount of money in a customer's account after all debts have been paid and all proceeds from sales received. In a margin account, a credit balance would include the proceeds held from a short sale which would be held in escrow for the borrowed securities for these sales.
This balance appears in the "Amount Due" column of your statement with a minus sign after the amount (i.e., $100-). Mayo Clinic may owe a refund to the patient or insurance plan, dependent upon review of the account.