Other forms: penes; penises
The penis is the male sexual organ. Boys and men and male animals all have penises.
Kids and teens often giggle at the word penis, but it's just a word for a body part like "arm" or "leg." Well, a penis is a little different, since it's the organ males use for sex and urinating. Some men take a lot of pride in their penis, which in slang is most often called a dick. However, if you're talking about a dick with a parent or teacher, or you're writing about one in a paper, penis is the word you want to use.