Software or programming that routinely issues a pop-up window or other form of visual interface asking the user to register a product, purchase an application or take another form of action. Nagware is typically used in reference to shareware, which typically offers a free-trial period of use for the application and then requests that the user purchase the rights to use the software indefinitely. The term gets its name from the idea that the reminders "nag" the user into taking some particular action as they will keep appearing until the desired action has been performed.
The reminder window that appears on the monitor often is referred to as the nag screen. Nag screens typically pop up when the application is activated and also when the user quits the program but can appear at any time during the application's use.