Action-adventure games with gameplay elements of the Metroid series and the Castlevania series.
Mechanics
Large interconnected map
A metroidvania often have a single large map or multiple large ones that one can travel between.
Powerups
As the game progress the player gains new weapons and abilities that make the player character stronger and able to defeat more powerful foes.
Unlock Areas
Progress in the game is locked down and the player need to find powerups to unlock new areas. It is often easy to understand what areas unlock when a new powerup is found. Like a blue door that can only be opened by freezing it with a ice cannon.
Shortcuts and Secrets
Besides the new areas the powerups also unlock faster ways to move between sections of the map and secret places with rewards.
Games
Axiom Verge - 2015
Super Metroid - 1994
Metroid - 1986