scrotum: The sac of skin at the base of the penis that holds the testicles (once descended).

sexual orientation: The statement of what gender (same, opposite, or both) a person is attracted to: homosexual, heterosexual, etc.

sexually transmitted disease; STD; sexually transmitted infection; STI: Any disease or infection that can be transmitted via sexual intercourse.

smegma: Cottage cheese-like substance that can form under an uncircumsised foreskin due to the combination of excretions and dead skin cells; if not cleaned, can lead to infection.

sonogram: A picture of a fetus taken in utero and made by sound waves.

spermicide: Chemical that kills sperm, generally applied topically before sexual intercourse as a contraceptive.

sterilization: Permanent method of preventing unintended pregnancy via surgery; either vasectomy (male) or tubal sterilization (women).

syphilis: A potentially deadly STD that can be hard to diagnose because it mimics other diseases (starting with canchres); can be transmitted through any intimate contact; treatable in most stages.