Noun

1. The portion of a communications link used for the transmission of signals from an Earth terminal to a satellite or to an airborne platform. An uplink is the converse of a downlink. An uplink or downlink is distinguished from reverse link or forward link.
2. By analogy and less formally, the communication path from a mobile device to a base station, a consumer to the network backbone, a client device to a server etc.

Adjective

1. Pertaining to data transmission from a data station to the headend.
2. Pertaining to GSM and cellular networks, uplink is the transmission path from a mobile station to the base station (cell site).