Obese and overweight are both medical terms.
Fat is a casual/colloquial term.
Overweight means to be over the recommended Body Mass Index ( BMI ) for a person's height.
Obese means to be extremely over the recommended BMI for a certain height.
(Obesity is considered medically dangerous.)
In polite conversation, you could refer to a person as overweight but only in the medical sense.
It is always better to use the word "large" or "larger".
For example: "Which person?"
"The larger gentleman on the left."
Americans loathe to talk about people's weight.
Perhaps because so many of us are on the large side. ;)
ell.stackexchange.com/users/14289/Taryn Teacher
These words have very specific meanings in medicine, but can have slightly different meanings in casual conversation outside of medicine.
In medicine:In the US, obese and overweight are terms used to describe people who weigh more than they should, according to current medical knowledge.
The exact weights can change as new medical research is done, but an obese person is a very overweight person.
Medically, fat is the type of tissue that makes a person overweight or obese.
In casual conversation:Usually obese means very overweight, but not always.
Both terms are used to describe someone who weighs to much when the speaker isn't trying to be insulting.
Casually speaking, fat can either mean the tissue causing you to be overweight ("I've got a ton of fat on my belly!"), or it can be a mildly insulting description of someone who weighs too much.
ell.stackexchange.com/users/5702/Karen
None of these are overt comparisons made of two or people.
They are simply modifiers.
He's fat.
He's overweight.
mean pretty much the same.
The second might imply that he needs to lose weight.
He's obese.
can refer to the scientific definition of obesity, or just be a remark similar to
He's really fat.
He's really overweight.
ell.stackexchange.com/users/None/
Yes, they differ in formality.
"Fat" is an informal term.
You wouldn't use it in, for example, a technical paper.
"Obese" is more formal.
"Overweight" is more polite, or at least, less impolite.
Calling someone "fat" would almost always be taken as an insult.
Saying he is "overweight" could be taken as a more neutral statement, perhaps expressing concern for the person's health, etc.
They also differ in degree.
"Overweight" could be used to describe someone who is only slightly over his ideal weight.
Like I weigh about 20 pounds more than my ideal weight.
I think most people would say that I was "overweight", but not "fat" or "obese".
"Fat" is a more middling term.
"Obese" is usually used only for people who are seriously overweight, like 100 pounds or more.
Note I'm not saying that there are concrete definitions, like up to 26.28 pounds is overweight and above
that is fat, the boundaries are vague.
ell.stackexchange.com/users/803/Jay
"Fat" is a simple word that every child learns.
It is surprisingly mean when used to describe a person.
Definitely avoid calling someone fat unless you intend to hurt them, and generally avoid calling someone else fat without thinking about it.
"Overweight" is the closest word to neutral.
I think it has a medical meaning regarding BMI being outside of the "healthy" range.
"Obese" has a medical meaning, and when used colloquially, people generally respect the medical meaning.
It means a BMI that is above 30.
It would mean "grossly fat."
ell.stackexchange.com/users/8045/djechlin
I agree with other answerers that "fat" and "overweight" are less formal and less clearly defined.
Obese, as pointed out, is a medical term, and there are defined degrees of obesity.
In the US, having a BMI between 30 and 35 is considered "obese" (Class I obesity).
Between 35 and 40 is called "severely obese" (Class II obesity).
Over 40 BMI is considered "morbidly obese" (Class III obesity).
In other countries this scale varies: in China the minimum BMI to qualify as "obese" is 28; in Japan, 25).
Until quite recently, at least in the US, only "morbid" (Class III) obesity was considered dangerous to one's health.
However, just last year, the AMA (American Medical Association) voted (against the recommmendations of their own select study committee) to declare obesity itself a "disease".
This I consider a political move; with that vote, they can now prescribe drugs and treatment to people who are overweight, but OTHERWISE PERFECTLY HEALTHY!
This opens up huge new marketing opportunities.
So a lot of money rides on the exact definitions of "obese", and of "disease", and on who gets to define them.
ell.stackexchange.com/users/15297/Brian Hitchcock
Could someone please tell me about the nuances of the adjectives:    Fat   Obese   Overweight     The only difference which comes to my mind is their formality degree, where 'fat' is the most informal and 'overweight' is the most formal and at the mid position stays 'obese'; do you confirm me?
Slight nuance between 'fat', 'obese' and 'overweight'
ell.stackexchange.com
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ell.stackexchange.com/questions/44917
ell.stackexchange.com/users/5652/A-friend
Obese  and  overweight  are both medical terms. Overweight could be used to describe someone who is only slightly over his ideal weight or BMI. Obese means to be  much higher than the recommended weight or BMI. Casually speaking, fat can either mean the fatty tissue itself or it can be an insulting term for someone who weighs too much. An obese person is a very overweight person according to BMI. Fat is a casual and insulting term, it can also refer to tissue.
An obese person is a very overweight person according to BMI. Fat is a casual and insulting term, it can also refer to tissue.