Which part of the computer is not an output device?

Output devices are those that relay information from the computer the environment or other devices: monitors, sound cards and speakers, etc.

Non-output devices are input devices (such as a keyboard or a microphone input). Not strictly output devices are combined input/output devices such as USB ports, serial ports, etc. Other devices, such as mass storage devices (hard disks, etc) are typically grouped separately from output devices, too.

Memory devices, processing units, and infrastructure devices such as the motherboard chipset, power supply, fans and mechanic parts are all other examples of parts of a computer that are not output devices.