The difference between being 'fat' and being 'overweight'
Being overweight simply means weighing too much - and what that means exactly, depends on the situation.
For instance, at the airport, your baggage can be 'overweight' if it weighs more than the official baggage allowance.
From a medical point of view, a person is overweight if they weigh more than is considered healthy for their height and age. However, such a person may not look heavy at all.
Remember, a person's weight can be contributed to by their bone-structure and muscle-mass too.
Fat, as a noun, is just a natural oily or greasy substance. It's also part of the make-up of our bodies. If we say a person is "fat" we mean s/he looks as if they have a lot of fat on their body.
It is more an aesthetic comment than a medical one.
A doctor would seldom tell you you were "fat" - they'd probably refer to as being one of the following instead:
- average weight
- obese (if you are extremely overweight)
To say someone is fat, on the other hand, is simply to make a comment about their appearance, based on your own perceptions - rather than any objective standard.