What is a demon in Potterverse

[Mark D.]

No: HPFGUIDX 67534

I was rereading PoA and had a question about Grindylows being 
defined as "water demons". I couldn't find demons listed in either 
Fantastic Beasts or the Harry Potter Lexicon. I always thought 
demons were creatures of the underworld (such as devils, hellspawn, 
fallen angels, followers of Lucifer...)

Can anyone come up with a suitable definition?