The
transfer of
control over
industry from
private hands to the
government.
Theoretically, this allows democratic control over said industry and promotes production that benefits the people rather than private profiteers.
The reality, of course, varies depending on just how democratic the government and/or society in question is.
Nationalization is generally considered a move toward socialism. The effect of nationalization will be radically different depending on whether the government is one of authoritarian socialism, libertarian socialism or false socialism (e.g. National Socialism, aka Nazism, which was a combination of "free market" capitalism and state capitalism).