Coitus is a word for sex, specifically vaginal sex that includes penetration and ejaculation. A couple has achieved coitus when they've had sex.
This word may sound a little unusual to you because it's an old technical term for sex. Very few friends sitting around talking about sex will describe it as coitus, unless they're talking about The Big Lebowski, a movie that makes humorous mention of this old-fashioned term. Coitus may remind you of copulation and fornication: two other fancy words for sex.
