A mobile app (in English, usually the short form, the app, an abbreviation for the subject term application, partly also the app) is an application software for mobile devices and mobile operating systems. Although the term app refers to any kind of application software, it is often equated with application software for smartphones and tablets in the German-speaking world. There are different types of mobile apps. The native apps that only work on one platform, and platform-independent web, hybrid, and cross-platform apps.

They are available for a wide range of applications ranging from simple utilities and fun applications with only one function to program packages with extensive functionality. For example, office applications, games, guidebooks, fitness apps, emulation of older home computers and programmable pocket calculators, as a help for diabetics and much more. Mobile apps are increasingly driving digital devices, such as cameras, action camcorders, or quadrocopters. The majority of these apps are free, a small part has to be bought, for mostly small amounts, in the respective App Store.

Even the first mobile phones often contained small applications such as calendars, pocket calculators or small games. These were designed by the manufacturer for the respective operating system, fixedly installed and not erasable. With the release of Java on mobile phones (Java ME), users were given the opportunity to download simple applications and games from their mobile operators or from the Internet via WAP (MIDlets), which were often platform-independent. A hobby programming scene developed, but also professional software houses offered such applications at a cost. Also on various PDAs applications could be installed. As predecessors of smartphone apps can be the Palm OS PDA applications, which consist mostly only of one file and after the transfer directly usable are. On Psion organizers, programs could be created with the device itself.