zygosis

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The union of gametes to form a zygote; conjugation.

Century Dictionary and Cyclopedia

1. n. Asexual intercourse of protoplasmic bodies, resulting in their confluence and coalescence; the process and result of conjugation in protozoans or other of the lowest organisms. See conjugation, 4.
2. n. [capitalized] [NL. (Förster, 1869).] A genus of hymenopterous insects.
3. n. In botany, conjugation; the fusion or union of two distinct cells or protoplasmic masses for reproduction. See conjugation, 4.

Wiktionary

1. n. The union of gametes to form a zygote; conjugation.

GNU Webster's 1913

1. n. The process of the merging of two gametes. Same as conjugation.