What Is It?
In the age of the NES, all games were "2D". This didn't refer to the flatness of the TV (what with the modern innovation of actual, 3D TVs), but to the graphics themselves. All sprites were 2D art, so all games using sprites were 2D games. These games could either be top-down (such as The Legend of Zelda), 3/4 isometric view (Equinox), or straight on (platformers like Castlevania, shooters like Gradius, puzzlers like Tetris, etc.).
This didn't really change until the innovation of basic 3D graphics. Although some games on the SNES sported pre-rendered polygonal graphics (3D graphics) set as sprites on the 2D plane (as popularized by the Donkey Kong Country series), the first "true" 2.5D game is commonly credited as Yoshi's Story for the Nintendo 64. That game featured lush, actively rendered polygonal graphics set against 2D platforming gameplay. From there, the term gained widespread use for any polygonal game featuring a 2D playing field.
What Castlevania Games Are 2.5D?
Currently only one title can be called a 2.5D game, Castlevania: The Dracula X Chronicles. Released in 2007, the game featured an enhanced remake of series favorite Rondo of Blood, with polygonal graphics but classic gameplay (some cutsceens were rendered in fully 3D, though).
The cancelled title, Castlevania: Resurrection, looked to have a 2.5D style to its game-play, although at times the linear path would have rotated towards the player, not making this a true 2.5D game (it was also cancelled, so it's not an official entry).
Although some 3D effects have been used in other games (such as rotating backgrounds or pre-rendered cut-scenes) as of yet, no other official Castlevania game has truly fit into the 2.5D mold. That said, the unofficial sequel, Bloodstained: Ritual of the Night is a 2.5D game using 3D graphics to tell a traditional 2D platforming adventure.