Video Games Encyclopedia

Metroidvania is a sub-genre of action-adventure games and/or platformers focused on nonlinear exploration and guided progression with a need to acquire key items to enter certain areas. The term is a partial blend of the names of the video game series Metroid and Castlevania, based on the template from Metroid (1986), Castlevania II (1987), Super Metroid (1994), and Castlevania: Symphony of the Night (1997).
These games usually feature a large interconnected world map the player can explore, although parts of the world will be inaccessible to the player until they acquire special items, tools, weapons, abilities, or knowledge within the game. Acquiring such improvements can also aid the player in defeating more difficult enemies and locating shortcuts and secret areas, and often includes retracing one’s steps across the map. Through this, Metroidvania games include tighter integration of story and level design, careful design of levels and character controls to encourage exploration and experimentation, and a means for the player to become more invested in their player character through role-playing game elements. While early examples were usually two-dimensional side-scrolling platform games, the term has since been applied to top-down and 3D games.
The first Metroid game in 1986 established principles of the non-linear platformer that were refined through multiple iterations, with Super Metroid in 1994 considered to have polished the style of gameplay core to Metroidvanias. Castlevania: Symphony of the Night in 1997 is considered the defining Metroidvania game, incorporating role-playing game elements from The Legend of Zelda series with non-linear traversal within the Castlevania series; most subsequent Castlevania games followed its approach and refined the genre.

Reference: Wikipedia