A boss (ボス bosu) is a significant and stronger enemy that is usually faced on its own, i.e. not in groups of enemies like other monsters.
Generally, boss fights will be longer even though they include only a single antagonist. This means that it is better to avoid using all Ethers and Potions on regular enemies to have a number at disposal upon the party's encounter with a boss.
Bosses as they appear chronologically in I Am Setsuna are,