(US: "tunneling") A networking technique used to carry data encoded in one protocol, A, over a channel using another protocol, B. Protocol A is said to be "encapsulated" in protocol B and treats B as though it were a data link layer. Tunnelling is used to get data between administrative domains which use a protocol that is not supported by the internet connecting those domains.
A historical example would be transmitting written text via Morse code - instead of having someone carry the text on paper, it is converted to (or encapsulated as) Morse code at one end and converted back to written text at the other.
A more recent example would be tunnelling IPv6 over an IPv4 network that does not support IPv6 natively. Tunnelling techniques such as 6to4 or 6rd are used to encapsulate IPv6 in the absence of native dual-stack support.