An incel, short for involuntary celibacy, is a person who feel as though they will never get to have sex. Such people have little to no game or are usually game denialists. Incel descibes many betas and most omegas.
According to Mystyckhan, the difference between an incel and a nice guy is that "Incels don't pretend that they're not potential rapists and sociopathic." ComradeMooseMøøse explains, "They in fact praise and worship those traits."