The anus is the hole in the middle of your buttocks. When you poop or fart, you're using your anus.
When you talk about your anus, you're using a formal, scientific word. Many kids and adults are more likely to use a slangy term like butthole or bunghole when discussing their anus, but if you're speaking to adults or writing a paper, anus is the word you want. The anus might seem like a funny part of the body, and maybe it is, but it's also incredibly important. Without an anus, you'd die.
