Alcohols, carboxylic acids, esters, alkanes etc are classes of organic compounds, not functional groups, no? (alkanes contain no functionl groups!)
In other words, a class is a type of compound, while functional groups specifically refer to a group of atoms (or more generally a bonding sequence).
However, it appears that both are mixed up very frequently. So are alcohols both the class and the functional group? Or is this an example of bad language?
Alcohol is a functional group that you are probably familiar with.
Instead, the table in the above link says "class" instead.
The sentences below sounds correct however (perhaps if a possessive is added: alcohol's, ether's, thiol's):
The alcohol functional group involves an oxygen atom that is bonded to one hydrogen atom
The ether functional group consists of an oxygen atom that forms single bonds with two carbon atoms.
The thiol functional group contains a sulfur atom bonded to a hydrogen atom.
I do see certain exceptions like ether referring to both the class, and the functional group (formally, alkoxy group). Perhaps this happens often for more complicated compounds; then it slowly encroached upon simpler ones....
Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995) (http://dx.doi.org/10.1351/pac199567081307)
10.1 - What are the 'functional groups' and 'classes' of organic compounds? https://youtu.be/tL_KSmwIm2I
An article provides nice intuition on the 'functional group approach' https://www.chem.fsu.edu/chemlab/chm1046course/functional.html