I’ve never liked using that term “antipope” so broadly - it is too negative (probably because it is immediately associated with the term ‘antiChrist’). I prefer “psuedo-pope” in some situations such as this (psuedo means ‘apparently similar but actually false’).
“anti” means ‘against’ or ‘opposed to’. There have been many antipopes (in the full sense of the word), but there have been a few psuedo-popes who were good and saintly men who really believed they had legitimate claim to the Papal office (but, alas, were mistaken - or tricked).