tunnel

Etymology

From Middle French tonnelle (“net”) or tonel (“cask”), diminutive of Old French tonne (“cask”), a word of uncertain origin and affiliation. Related to Old English tunne (“tun; cask; barrel”). More at tun.

noun

1. An underground or underwater passage.
   In 1865 an outfit called the East London Railway Company bought the Brunel tunnel for £800,000, and in 1869 they opened a railway through it. 2012, Andrew Martin, Underground Overground: A passenger's history of the Tube, Profile Books, page 90
2. A passage through or under some obstacle.
   There are more than 1,500 railway tunnels in Britain and the majority are still in use, carrying working tracks beneath Britain's most inconvenient geographic features. August 26 2020, Tim Dunn, “Great railway bores of our time!”, in Rail, page 42
3. A hole in the ground made by an animal, a burrow.
4. (computing, networking) A wrapper for a protocol that cannot otherwise be used because it is unsupported, blocked, or insecure.
5. A vessel with a broad mouth at one end, a pipe or tube at the other, for conveying liquor, fluids, etc., into casks, bottles, or other vessels; a funnel.
6. The opening of a chimney for the passage of smoke; a flue.
7. (mining) A level passage driven across the measures, or at right angles to veins which it is desired to reach; distinguished from the drift, or gangway, which is led along the vein when reached by the tunnel.
8. (figurative) Anything that resembles a tunnel.
   Especially in the Eden Valley, trees create what is almost a green tunnel (particularly in summer). October 20 2021, Mark Rand, “S&C: a line fit for tourists... and everyone?”, in RAIL, number 942, page 43

verb

1. (transitive) To make a tunnel through or under something; to burrow.
   The 1955 Act gave powers for compulsory acquisition of "easements", or permission to tunnel beneath dwelling houses instead of, as had previously been necessary, following approximately the course of surface roads. 1962 October, “London gets its Victoria tube”, in Modern Railways, page 258
   The 6.5km route is agreed from a junction with the relief lines of the Great Western main line to the west of Slough, the new link would tunnel under the M25 to reach Heathrow's Terminal 5 station, where space has been set aside to accommodate services from the west. 2019 October, Ruth Bagley tells James Abbott, “Crunch time for Heathrow western link”, in Modern Railways, page 74
2. (intransitive) To dig a tunnel.
3. (computing, networking) To transmit something through a tunnel (wrapper for insecure or unsupported protocol).
4. (transitive, medicine) To insert a catheter into a vein to allow long-term use.
5. (physics) To undergo the quantum-mechanical phenomenon where a particle penetrates through a barrier that it classically cannot surmount.