Privatization means transferring something into private sector, while nationalization means moving it under control of government (or, euphemistically, into public sector). They are opposites.
Privatization is usually done as voluntary trade, where interested private investors can purchase the privatized good from the government. However, nationalization is often forceful, and private businesses are simply robbed of their property by a government decree.
It’s also worth noting that privatization means the government offers something for sale to the citizens, despite the fact that the same citizens have already paid for those goods in their taxes. Examples of just privatization, where citizens receive their shares for free, are very, very unusual.