Video Games Encyclopedia

A platformer (also called a platform game, and sometimes a jump ‘n’ run game) is a subgenre of action game in which the core objective is to move the player character between points in an environment. Platform games are characterized by levels with uneven terrain and suspended platforms that require jumping and climbing to traverse. Other acrobatic maneuvers may factor into the gameplay, such as swinging from vines or grappling hooks, jumping off walls, gliding through the air, or bouncing from springboards or trampolines.
The genre started with the 1980 arcade video game Space Panic, which has ladders but not jumping. Donkey Kong, released in 1981, established a template for what were initially called “climbing games”. Donkey Kong inspired many clones and games with similar elements, such as Miner 2049er (1982) and Kangaroo (1982), while the Sega arcade game Congo Bongo (1983) adds a third dimension via isometric graphics. Another popular game of that period, Pitfall! (1982), allows moving left and right through series of non-scrolling screens, expanding the play area. Nintendo’s flagship Super Mario Bros. (1985) was a defining game for the genre, with horizontally scrolling levels and the player controlling a named character—Mario, which became Nintendo’s mascot. The terms platform game and platformer gained traction in the late 1980s.
During their peak of popularity, platformers were estimated to comprise between a quarter and a third of all console games. By 2006, sales had declined, representing a 2% market share as compared to 15% in 1998. In spite of this, platformers are still being commercially released every year, including some which have sold millions of copies.

Reference: Wikipedia