J.K. Rowling defined jinxes as "spells whose effects are irritating but amusing."
As with most spell names, this classification is not hard and fast; for instance, Tom Riddle's Jinx on the Defence Professorship of Hogwarts was neither a minor effect, nor amusing, nor truly irritating. Indeed, it proved lethal for its targets on several occasions. Less strikingly, other jinxes, while admittedly "amusing", have also demonstrated lethal consequences, such as the tree animating jinx. Even though Rowling herself specified the differences between curses, hexes, and jinxes, she states that "Within the Potter world, the boundaries are flexible and I imagine that wizards may have their own ideas." This shows that the categorization of dark charms is not set and stone, and may sometimes be used interchangeably. This explains why some spells are known by multiple names, like why the Slug-vomiting Charm is a curse, and why the Jelly-Legs Curse is also sometimes known as the Jelly-Legs Jinx. This also explains why some curses, such as the Leg-Locker Curse, Jelly-Legs Curse and Full Body-Bind Curse, which don't cause any lasting injury, seem to fall more under the definition of a jinx or hex, while Voldemort's Jinx on the post of Defence Against the Dark Arts teacher has had serious, lasting consequences for several of those affected, which would seem more like a curse.