Pub 17

Alimony is a payment to or for a spouse or former spouse under a divorce or
    separation instrument. It does not include voluntary payments that are not made
    under a divorce or separation instrument. 

    Alimony is deductible by the payer and must be included in the spouse's or former
    spouse's income. Although this chapter is generally written for the payer of the
    alimony, the recipient can use the information to determine whether an amount
    received is alimony. 

    To be alimony, a payment must meet certain requirements. Different requirements
    apply to payments under instruments executed after 1984 and to payments under
    instruments executed before 1985. This chapter discusses the rules for payments
    under instruments executed after 1984.