Cost Basis
Cost basis is what you originally paid for an investment, including fees and reinvested dividends. It's the number the IRS subtracts from your sale price to calculate your taxable capital gain (or loss) — so tracking it accurately can save real money at tax time.
Brokerages report basis for most holdings, but transfers and inherited assets can muddy it. Inherited investments get a 'step-up' to their value at death, often wiping out decades of gains — one of the most valuable quirks in the tax code.