In astronomical parlance, apparition is the term that refers to the appearance of periodic comets. Periodic comets are classified as those comets whose orbital period does not reach 200 years, or those who have been seen in more than one perihelion passage. They are also called short-period comets. The term periodic comet is also used to mean any comet that is marked by a periodic orbit, regardless of it being greater or less than two centuries.
Popular comets in human history are often signalled by apparitions, and are often named after the persons who were lucky enough to have seen them. Halley’s Comet, Finlay’s Comet, and Neujmin Comet are just some of the examples. However, there are also periodic comets whose name is from the person who actually calculated their orbit, and examples are 2P/Encke and 27P/Crommelin.
It is not always easy to tell the next apparition of a periodic comet, and this is because the orbit of such comets is also not easy to measure exactly. Possible space perturbations coming from various planets might mess up the celestial path. Few comets were actually lost because of their orbits getting affected by non-gravitational factors such as gas release and some other materials that form and define the coma and tail of a comet.