"Zeo" originally was to stand for "Zordon of Eltar's Odyssey", in reference to the Power Rangers' alien mentor. It was changed to just an alien term for a power crystal, which was introduced in the latter part of Mighty Morphin Power Rangers (1993).
It's the first Power Rangers series in which a Ranger from a previous team (Jason Lee Scott, the first Red Ranger from Mighty Morphin' Power Rangers and the Gold Ranger in Power Rangers Zeo) joins to the team with his successor (Rocky DeSantos, the second Red Ranger from Mighty Morphin' Power Rangers and the Blue Zeo Ranger in Power Rangers Zeo).
The Gold Ranger was originally intended to be Ryan Steele, the protagonist of V.R. Troopers (1994), with Saban going so far as to have actor Brad Hawkins voice the morphed Gold Ranger. The idea of Billy becoming the Gold Ranger was also floated, but eventually an entirely new character (Trey of Triforia) was created before the powers were passed on to Jason.