A mobile device (or handheld computer) is a computer small enough to hold and operate in the hand. Typically, any handheld computer device will have an LCD or OLED flatscreen interface, providing a touchscreen interface with digital buttons and keyboard or physical buttons along with a physical keyboard. Many such devices can connect to the Internet and interconnect with other devices such as car entertainment systems or headsets via Wi-Fi, Bluetooth, cellular networks or near field communication (NFC). Integrated cameras, the ability to place and receive voice and video telephone calls, video games, and Global Positioning System (GPS) capabilities are common. Power is typically provided by a lithium battery. Mobile devices may run mobile operating systems that allow third-party apps specialized for said capabilities to be installed and run.

Early smartphones were joined in the late 2000s by larger, but otherwise essentially the same, tablets. Input and output is now usually via a touch-screen interface. Phones/tablets and personal digital assistants may provide much of the functionality of a laptop/desktop computer but more conveniently, in addition to exclusive features. Enterprise digital assistants can provide additional business functionality such as integrated data capture via barcode, RFID and smart card readers. By 2010, mobile devices often contained sensors such as accelerometers, magnetometers and gyroscopes, allowing detection of orientation and motion. Mobile devices may provide biometric user authentication such as face recognition or fingerprint recognition.

Major global manufacturers of mobile devices are Apple, Samsung, Huawei, Xiaomi, Sony, Google, HTC, LG, Motorola Mobility and Nokia.