Average cost

What you paid per share on average across all your buys: the total cost divided by the shares.

Average cost is the cost basis divided by the number of shares. Buy 10 shares at $50 and 15 at $40 and you have paid $1,100 for 25 shares — an average of $44, not $45, because more shares were bought at the lower price.

A later buy changes the average; a sale under the average-cost method does not — the shares sold leave at the average, and the ones left keep it.