A frilly-looking sort of skirt, made of tulle and worn by ballerinas and cool people. One can tell when it is worn by a ballerina because the person in question will usually also be wearing some form of dance shoe and have their hair up in a strict bun. When a cool person wears a tutu, one can tell because others are always gazing upon them in unabashed admiration and desire. Cool people in tutus can often (but not always) be found fighting crime, assassinating terrorists, and helping little old ladies cross the street.
Ballerina: Look at me dancing around in my little tutu!
John: Emma is wearing a tutu. She's not a ballerina, so that means she must be a cool person. I wish I had a tutu.
Passerby: Don't we all.