Following are the basis of comparison here in the UK:
 Size 10 is a slim, Size 12 being the average, 8 and below are skinny ,14 is being big and 16 is overweight 18+ is obese.

I think I weigh a lot I am size 10-12 but can fit into size 8s (sometimes) but I weigh a lot then what people think. I think  it’s not the size or weight that makes you fat but what you wear and how you carry it!
 

It is hard to say.
My BMI states that I am obese (I have a BMI of 31.4) yet I wear size 14-16 (uk size) clothes. I consider myself fat but I wouldn't say I was obese and neither do I wear plus size clothes.

I am above my bmi, considered obese and wear a double digit, but no one ever believes me when i tell them i'm a 12/13 (us). i never looked my weight, always smaller. i think it has to do more with how you carry your weight then your actual weight.

Medically, it is based on BMI (Body Mass Index). It depends on your height, and your frame and your relative muscle mass vs total weight.

Overweight is BMI of 25-29.9
Obese is BMI of 30 or greater


I would certainly consider obese people as ""fat"" but not all overweight people are fat.