Definition
The account balance is the posted cash value after completed deposits, withdrawals, fees, interest, rewards, and realized trading results have been recorded.
In market context
An account balance is a ledger figure, not necessarily the amount that can be withdrawn or committed to a new trade immediately. Pending transfers, reserved funds, unsettled activity, and open-position requirements can make available cash lower than the balance. Comparing the balance with account equity and available cash helps explain whether a change came from posted transactions or current market movement.
Source
Use the primary source for fuller regulatory or market context.
