A spermatozoon (alternate spellings spermatozoan, spermatozoön; plural spermatozoa) is a motile sperm cell, or moving form of the haploid cell that is the male gamete. (A non-motile sperm cell is called a spermatium.) A spermatozoon joins an ovum to form a zygote. (A zygote is a single cell, with a complete set of chromosomes, that normally develops into an embryo.) The term spermatozoon comes from the ancient Greek word σπέρμα (seed) and ζῷον (living being).
Sperm cells contribute approximately half of the nuclear genetic information to the diploid offspring. In mammals, the sex of the offspring is determined by the sperm cell: a spermatozoon bearing a Y-chromosome will lead to a male (XY) offspring, while one bearing an X-chromosome will lead to a female (XX) offspring (the ovum always provides an X-chromosome). Sperm cells were first observed by Anton van Leeuwenhoek in 1677.