What are “rare” words and how they are chosen?

The blue R symbol next to the answers means a rare word has been found. Rare words are decided by the number of times they've been played, so if a word hasn't been played very often, it will be considered a rare word. Mind the fact that sometimes the rare status of a word may vary depending if it’s written on its singular/plural form. They will also serve as a tiebreaker in case both players got the same amount of points in the round.