An ordered set of labels appended to a packet that enables it to implicitly carry information about more than one FEC that the packet belongs to and the corresponding LSP that the packet may traverse in regards to the Multiprotocol Label Switching technology. A label stack enables MPLS to support a routing hierarchy (one label for the EGP and one label or the IGP) and to aggregate multiple LSPs onto a single "trunk" LSP.
Other applications include, VPNs and dynamic QoS.