This is not the fault of modern media for once, but it really confuses me how arbitrarily the terms Titan is applied to various figures. I mean, what on earth makes Leto a Titan? Do tell, Nonnos. Why are Hekate and Kirke called Titans? What even is a Titan? Is it a matter of generation? Doesn't seem so. Of parentage? Doesn't seem so. Of power or importance? Doesn't seem so since Nonnos calls Lelantos and Aura Titans and they play no significant role in Greek mythology. Of age/antiquity? Of convenience? But whatever, such details were clearly not particularly important for ancient writers who frequently mixed up the Titanomachy and Gigantomachy. Fine, moving on.
However, modern media and discussions on the subject do very much accentuate the issue by making Titans basically a different species than the gods or at least meaningfully different from them and calling everyone and their mother a Titan(ess). I've seen Gaia and Ouranos called Titans, also Nyx, Erebos, Typhon, Aphrodite. Everyone insists on calling Metis a Titaness even though not once in all extant Greek mythology is she ever described as such or listed among the Titans; she is called simply θεὰ "goddess" in Hesiod's Theogony and elsewhere she is just "daughter of Okeanos". Why are the children of Krios and Eurybie called Titans? Ok, call all the most notable children of the Titans also Titans as the theoi site does even though many of those are never called Titans anywhere, but how come then that Kronos and Rhea are the only Titans not to beget Titan children?
Anyways, the idea of Titans being the sons of Gaia and Ouranos, the former gods who ruled over the world before the Olympians and now live beneath the earth in Tartaros as they appear in various Archaic texts would be perfectly coherent and logical, and fit in with this passage from Hesiod's Theogony that singles out the sons of Ouranos as Titans: "But these sons whom he begot himself great Heaven used to call Titans (Strainers) in reproach, for he said that they strained and did presumptuously a fearful deed, and that vengeance for it would come afterwards." It would also not be unreasonable for the sisters of the Titans to share their title occasionally despite not taking part in either their deeds or their ultimate fate. Helios as Titan makes sense since he is often identified with his father Hyperion. Atlas is an enemy of the Olympians so him being considered one of the Titans also makes sense. But the rest are pretty random.
Honestly I could probably deal just fine with the inconsistency and arbitrariness if I didn't constantly see it claimed or implied that being a Titan or a so-called second generation Titan is somehow different from being a god.