The term "annuity", as used in financial theory, is most closely related to what is today called an "immediate annuity". This is an insurance policy which, in exchange for a sum of money, guarantees that the issuer will make a series of payments. These payments may be either level or increasing periodic payments for a fixed term of years or until the ending of a life or two lives, or even whichever is longer. It is also possible to structure the payments under an immediate annuity so that they vary with the performance of a specified set of investments, usually bond and equity mutual funds. Such a contract is called a variable immediate annuity. See also life annuity, below.