zygosis

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The union of gametes to form a zygote; conjugation.

from The Century Dictionary.

• noun 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.
• noun [capitalized] [NL. (Förster, 1869).] A genus of hymenopterous insects.
• noun In botany, conjugation; the fusion or union of two distinct cells or protoplasmic masses for reproduction. See conjugation, 4.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Biol.) The process of the merging of two gametes. Same as conjugation.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biology The union of gametes to form a zygote; conjugation.