Any physical piece of a computer or network system, such as a monitor, keyboard, ethernet cable, etc.

Merriam-Webster Online Dictionary
hardware (noun)
1.
ware (as fittings, cutlery, tools, utensils, or parts of machines) made of metal
2.
major items of equipment or their components used for a particular purpose as - educational hardware
a) military equipment
b) the physical components (as electronic and electrical devices) of a vehicle (as a spacecraft) or an apparatus (as a computer)
3.
an award (as a trophy, medal, or cup) given in a sports competition