Unpopularity is the state of not being liked by most people — the noun for being unpopular. It is used about people (a boss, a teacher, a president), and just as often about things: a policy, a tax, a rule, a decision, a new design. That second use is where you meet it most, in news and serious writing: 'the growing unpopularity of the new tax', 'the government's unpopularity in the polls'. It is uncountable and fairly formal; in speech people usually rearrange the sentence and use the adjective instead — 'nobody likes him' or 'the plan was really unpopular' rather than 'his unpopularity'. Two shades worth knowing. It can be measured and public, almost a number, as when a leader's unpopularity shows up in surveys. Or it can be personal and painful, as when a child suffers from unpopularity at school — and there English often softens it to 'he doesn't have many friends', because the noun sounds cold when it is about a real person's life. Note also that unpopularity means actively not liked, which is stronger than simply not famous; a singer nobody has heard of is unknown, not unpopular.