Packet switching
Splitting data into packets routed individually, instead of reserving a dedicated circuit.
The telephone network was circuit-switched: a call tied up a line continuously, silence included. Paul Baran and Donald Davies independently formulated the alternative in the 1960s — chop the traffic up, address each piece, and let the network fill the gaps with other people's packets.
The gain is statistical multiplexing: the same link carries far more concurrent flows because nobody reserves capacity they are not using. The price is variable delay and possible packet loss, which is exactly what TCP exists to handle.