[¦pir·ē·əd ‚par·ə′lel·ə‚gram] (mathematics) For a doubly periodic function ƒ (z) of a complex variable, a parallelogram with vertices at z0, z0+ a, z0+ a + b, and z0+ b, where z0 is any complex number, and a and b are periods of ƒ(z) but are not necessarily primitive periods.