Fixed annuity definition is divided in two parts.

Fixed; Placed or fastened securely; made firm in position.
           Definite and unalterable.
           Permanent.
           Prearranged as to outcome or decision.
           Not volatile.
           To make firm or secure. To be firm or stable.

Annuity; Income.
             An income paid monthly, quarterly, simi-annually
             or annually.
             The right to receive such an allowance or the duty
             of paying it.

National Association of Insurance Commissioner
Buyer's Guide To Fixed Annuities.