All areas in an Open Shortest Path First (OSPF) autonomous system must be physically connected to the backbone area (Area 0). In some cases, where this is not possible, you can use a virtual link to connect to the backbone through a non-backbone area...
http://www.ebrahma.com/2013/10/how-to-configure-virtual-links-in-ospf/Labels: backbone-area, non-backbone-area, Open-Shortest-Path-First-(OSPF), OSPF-database-in-a-virtual-link, OSPF-virtual-link, Virtual-Links, Virtual-Links-in-OSPF