The BMI (Body Mass Index), while far from perfect - mainly because it can't tell the difference between relatively heavy muscle and relatively light (density wise) fat, so a very fit and very strong person can wrongly be classified (and in the opposite direction too) - is still the standard recognised by the WHO, by the NHS in the UK, etc. It's just calculated by body mass divided by the square of the body height, using kg and m.

And then there are specific names for ranges of BMI, which include "Normal range" for 18.5 to 24.9, "Overweight (Pre-obese)" for 25-29.9, and then three classes of "Obese" above 30 as well as three "Underweight" types below 18.5.

Generally this is simplified down to "Underweight", "Normal", "Overweight", and "Obese", with the following words after "Normal" and "Overweight" being implied, and with the multiple levels of underweight and obesity ignored unless discussing someone light or heavy enough that it's worth considering the differences between "Obese (Class I)" and "Obese (Class III)", or between "Underweight (Mild thinness)" and "Underweight (Severe thinness)" - all still based purely on the BMI number.

Sorry for the long winded answer, but even more here: https://en.m.wikipedia.org/wiki/Body_mass_index

Weirdly it seems like this study used the same logic but set non-standard boundaries (27.5 rather than 30 as the point between Overweight and Obese).