Fat is how much flab you have on your body. A person can have very little muscle mass and lots of fat on their frame, and still look "thin" with clothes on.
But if they take their clothes off, their body composition looks bad because the mass they do have is almost all fat.
You can be "thin" but have 30% body fat.
People get too caught up with looking at numbers on a scale. That is just as silly as the BMI.
The most accurate way to measure how fat somebody is is to use a body fat caliper.
Bunch of morons in this thread who have never worked out a day in their life trying to tell me.
Look at this dude:
With a shirt on, you'd say, hey, that guy is just a regular thin guy.
But when he takes his shirt off, he's very out of shape. He's got about 20-25% body fat and no muscle mass.
He's thin and fat at the same time.