When was the term "tank" first used as a name for armoured fighting vehicles?

Great Britain was developing armoured vehicles that were described as "Caterpillar Machine Gun Destroyers" or "Land Cruisers" during the First World War. A term used by the committee in charge of development called them "landships" but it was thought to change the name for secrecy. A meeting in late 1915 of representatives from the different offices and committees that were involved in the project decided on the term "water carrier". So the "Committee on Landships" would be named "Committee on Water Carriers" instead.

An issue arose, British government offices, committees and departments are often known and referred to by their initials, so the "Committee on Water Carriers" would be called "W.C. Committee" for short. W.C. is a British abbreviation for toilet ("water closet"), a synonymous term "tank" was used in place of water carrier. The committee was then known as "Tank Supply" or "T.S." Committee. The secret water carriers were then referred to as tanks.