Cupronickel is an alloy of copper, nickel and stengthening impurities.
Cupronickel does not corrode in seawater, because its electronegativity is adjusted to be neutral with regard to seawater. Because of this it is used for marine hardware, and sometimes for the propellers, crankshafts and hulls of premium tugboats, fishing boats and other working boats.
The most ubiquitous use, from the point of view of the average person, is that most of the silver-coloured modern circulation coins are Cupronickel. A typical mix is 75% copper, 25% nickel, and a trace amount of manganese. In the past true silver coins were debased with cupronickel.
It is used in thermocouples, and a 55% copper/45% nickel alloy is used to make very accurate resistors.
See also bronze (copper alloyed with tin), brass (copper alloyed with zinc), and nickel silver (another group of copper-nickel alloys).