Adjusted Cost Base (often abbreviated as ACB) is the tax-reportable cost of a security held in a non-registered Canadian account. When you sell, your capital gain (or loss) is calculated as the proceeds minus your ACB. Getting ACB wrong means overpaying or underpaying tax on every sale.
ACB isn't just what you paid. It also includes commissions on the buy, reinvested distributions (DRIPs), return-of-capital adjustments from ETFs, and spinoff / stock-split adjustments. Every buy, every reinvested dividend, every corporate action changes your ACB.
Because ACB is calculated per security across your whole (non-registered) portfolio (not per account), you have to consolidate identical shares across Questrade, Wealthsimple, TD Direct, and any brokerage you've used. A single ETF held in two accounts has one blended ACB.
WealthWatch tracks ACB automatically. Every buy, sell, DRIP, and corporate action updates your ACB in real time, so when you sell, the realized gain is already computed for your tax return.