The Dictionary of Insurance Terms and Definitions
An annuitant is any person who receives the benefits (i.e. pension) from an annuity.
Usually, the annuitant is also the annuity owner, but this is not necessarily the case. The owner is in charge of paying premiums during the annuity's accumulation phase and then determining when to end the accumulation phase (which triggers the start of the pension). A person might well perform these tasks without designating himself/herself the beneficiary of the annuity's pension.
The size of the pension payments are determined, in part, by the life expectancy of the annuitant at the end of the accumulation phase.