An uplink is that part of the communication link where signals from an earth terminal are sent to a satellite or some other type of airborne platform, hence the "up" portion of the term.
The term is commonly used in different areas of telecommunications such as cellular networks and computer network where the communication is coming from a lesser station up towards a larger portion of the network.
In satellite communication, uplink means that data are being passed from any earthbound terminal or device up towards the satellite.
The opposite of this is a downlink, where the communication is coming from the satellite towards any earthbound device.
In cellular networking, an uplink is seen from the user perspective as it pertains to the communication link of any cellular device sending data towards the cellular base station.
For example when sending a text message, the phone creates an uplink with the base station in order to send the text message. The inverse is receiving a text message through a downlink.
The term is used in the same way for computer networking. Nodes or edge terminals create uplink connections to send data towards the network core. This is also known as an upstream connection, or simply uploading. Downloading is the inverse.