Also called: basis
What you paid for an investment, including reinvested dividends. When you sell, only the amount above basis is a taxable gain.
In a taxable brokerage account, the higher the basis relative to the balance, the less tax a withdrawal triggers. RetIQ tracks one pooled basis for all brokerage accounts: it rises with contributions and reinvested dividends and falls proportionally with withdrawals.
Pre-tax accounts generally have no basis, so every dollar withdrawn is ordinary income.