Your hero believes something that is considered quite strange by everyone else. Minor Delusions are harmless or the character generally keeps it to himself (the government puts sedatives in soft drinks, dogs can talk, we’re all just characters in some bizarre game, etc.). With a Major Delusion, he expresses his view on the situation frequently and it can occasionally lead to danger (the government is run by aliens, hospitals are deadly, I’m allergic to armor, zombies are my friends.