The notional value of securities you can purchase right now given cash, margin entitlement and current positions — not the same number as your account equity.
Margin and leverage. A $5,000 deposit can control a $100,000 position, which is 20:1 leverage. Because the loss is measured on the full $100,000, a 2.5% move against you halves the deposit and brings a margin call, and a 5% move uses all of it.
Buying power is not your cash. It is cash plus whatever credit the margin rules extend: typically 2x equity for an overnight stock account under regulation-t, 4x for a pattern day trader intraday, and far more in futures where margin is a performance bond.
It changes constantly. Unrealised losses cut it, concentrated positions can carry house requirements above the regulatory minimum, and options positions consume it according to their risk profile rather than their cost.
Two cautions. First, using all available buying power means a modest adverse move triggers a margin-call or forced-liquidation at the worst possible moment. Second, buying power is a broker constraint, not a risk rule - your max-open-risk limit should bind long before it does.