What is the meaning of uplink and downlink?

In satellite telecommunication, a downlink is the link from a satellite down to one or more ground stations or receivers, and an uplink is the link from a ground station up to a satellite .

Since a mobile station (cellphone) cannot afford to transmit at high power to compensate for this loss, it must transmit on a lower frequency as a lower frequency presents lesser free space path loss. Therefore, mobile-to-base station (uplink) frequencies are lower than base station-to-mobile(downlink) frequencies.

Definition of uplink 1 : a communications channel for transmissions to a spacecraft or satellite also : the transmissions themselves. 2 : a facility on earth for transmitting to a spacecraft or satellite.