Sinking fund
Money set aside a little at a time for a known future expense, so the cost is spread across the months before it falls due instead of landing in one.
A sinking fund turns a lumpy cost into a level one. An annual insurance premium, a holiday, new tyres, a roof that will need replacing: each has a rough amount and a rough date, so dividing the amount by the months remaining gives a figure to set aside every month. When the bill arrives, the money is already sitting there and nothing else in the budget has to move. The savings goal calculator runs the same arithmetic, including any interest the balance earns on the way.
The name comes from bond finance, where an issuer sets cash aside on a schedule to retire debt steadily rather than meeting the whole principal on one day. The household version borrows the mechanism exactly: pay a known future obligation in advance, in instalments, without borrowing to do it. Most people run several at once, one per goal.
The mistake is running a single pot for everything, emergencies included. A sinking fund is for expenses that are known and dated; an emergency fund is for the ones that are neither. Mixed together, the first surprise repair drains the money earmarked for the insurance renewal, and the renewal ends up on a card. Separate balances, even inside one account, keep each pot honest about its job.