Adventure is a genre of video games, prevalent since the days of the Atari system and the eponymous game "Adventure", and probably most popular during the era of 8-bit and 16-bit gaming. Like most genre definitions, what constitutes an "adventure" game is somewhat open to interpretation. The adventure game takes in the large middle ground between platformers and RPGs. An adventure game will involve real time movement of a character, but will not primarily depend on rapid reflexes. It will have an inventory of items, perhaps some form of interaction with other characters, and some ability to choose the route your character takes.
This is, of course, a very wide net, and games that could be described as "adventure" games range from Super Mario Brothers 3, which is a platformer with some control over the route and a basic inventory system, to a game like Terranigma,a complicated, sweeping RPG which included some dexterity-intensive real time combat. Within the loose confines of this definition, "adventure" was the default genre during the 8-bit and 16-bit era. Some of the most popular franchises of that era, such as Super Mario Brothers (starting with the third game), The Legend of Zelda, Metroid and Megaman, were all games in the adventure mold.