For questions about routing, the process of selecting to which network path traffic is sent. Consider using a more specific tag if your question is about a routing protocol like BGP or OSPF.
Routing is a process performed by a router to transfer packets from a network to another one. It consists in determining the correct destination network based on known routes and sending out the packet on the corresponding interface.
Nowadays the term routing almost always relates to IP routing - i.e. routing IP packets, but it could refers to other -generally deprecated- protocols like Novell IPX/SPX.