noun One of a series of parts, whether equal or unequal to the other parts of the series, of a given entity or a given process, which part presents or is presented at a particular scheduled interval.
noun Any of a series of portions of a debt or sum of money, which portions may or may not be equal in amount (depending in part on whether the interest rate is fixed or variable), and payment of which takes place at regular intervals toward satisfaction of the total. Payments are generally mensual, quarterly, triannual, biannual, or annual.