EMZETT.Line
In short: The physical or logical medium over which data is transmitted between two points — from classic copper cable through fibre to a logical connection inside a tunnel.
In more detail: “Line” is used both for the real physical transmission path (cable, radio link) and for a logical, dedicated connection established over a shared network (e.g. a leased line or an SSH tunnel). Properties of a line such as bandwidth and latency directly determine how much data can be transmitted in what time.
In Depth
Physical lines differ greatly in their properties: copper cables (e.g. coaxial cable or twisted-pair cables such as the Cat cable family) are cheap and easy to lay, but susceptible to electromagnetic interference and limited to short to medium distances. Fibre (fibre) transmits light instead of electrical signals, is immune to electromagnetic interference and achieves much higher bandwidths and longer distances — but it’s more expensive to buy and install. Radio links (wireless/Wi-Fi) dispense with a physical medium entirely, but in return are more prone to interference and usually slower than a comparable cable connection.
With logical lines (e.g. a leased line or a tunnel), the “line” isn’t a separate physical route, but a dedicated, often encrypted connection laid over shared physical infrastructure (e.g. the public internet). To the endpoints it behaves as if it were a separate, exclusive line — even though the underlying packets share the physical medium with other traffic.