Colloquially, gameplay is a measure of fun in a video game; more precisely, gameplay is the degree and nature of the interactivity that a game includes, i.e., the challenges a game presents to players and the actions that players may take in response.
Gameplay is the primary criterion for categorizing video games into genres. For example, a game that is set in first-person perspective and allows a player's avatar to fire weapons is a first-person shooter.