I do not think that there is a body of experts who agreed on just how
much fat or carbs constitutes a "low fat" or "low carb" diet.

Our government suggests that we eat under 30% of calories from fat,
so, it is not unreasonable to use that as a definition of low fat.

As for low carb, there is no cutoff amount, but then, it would depend
on the person anyway. I would consider anything below 20% of calories
carbs, to be low carb.

Low calorie simply means eating fewer calories than required to
maintain weight.