A low-cost Linux and ARM-based computer on a small circuit board sponsored by the charitable Raspberry Pi Foundation in the UK. (www.raspberrypi.org). In 2011, the first Raspberry Pi prototype was the size of a flash drive with USB on one end and HDMI on the other. In 2012, the circuit board expanded to roughly 3x5 inches, and the USD $35 Pi 1 Model B was the first commercial product. The Model A followed soon after along with several improvements to the Model B. The $5 Pi Zero and $10 Pi Zero W (Wi-Fi and Bluetooth) are smaller units with fewer ports.
Pi models can be used as a desktop computer, the heart of a media center and other applications. They give hobbyists and tinkerers an economical computer to experiment with, and although they were first touted as a learning tool, Raspberry Pi's are used for myriad purposes including Internet of Things (IoT) devices. See computer on a stick
and Internet of Things
The Original Pi - The Model B
Priced at USD $35, the 3.3x5.3" Raspberry Pi board can be used in desktop and portable applications. The $25 Model A has only one USB port and no Ethernet, and the $5 Pi Zero and $10 Pi Zero W are even smaller with fewer I/O ports. (Image courtesy of the Raspberry Pi Foundation.)