Nomenclature
In the 19th-century (and before) British Army, regiments of infantry raised several battalions, which were often deployed separately. This practice remained into the modern era—in the First World War, it was common to see twenty or more battalions with a single regimental title. However, this practice did not hold for the cavalry regiments, which traditionally were only of limited size; in the modern era, this meant that each regiment would only constitute one battalion.
As a result, it became traditional for a battalion-level unit of cavalry to be referred to as a "regiment". This was not as confusing as it may seem, since where other armies would use "regiment" for a unit of two to four battalions, the British Army used "brigade". Hence, an infantry brigade could consist of three battalions of infantry, but a cavalry brigade of equivalent size would have three regiments.
In the inter-war period, the British Army began to mechanise, with cavalry regiments giving up their horses in favour of armoured cars or light tanks - the first to do so was the 11th Hussars, in 1928; the last was the Royal Scots Greys in 1941. As a result, it became common to refer to any armoured unit as a "regiment" rather than a "battalion" - the 11th Hussars were not merely an armoured-car battalion, but the whole of the regiment. In 1945, this usage became formal; all armoured battalions were henceforth referred to as regiments.
The Royal Tank Regiment is itself a regiment of the British Army, part of the Royal Armoured Corps (RAC). However, as a result of the above, both of its "battalions" are formally titled regiments. This can cause some confusion, with the regiment currently being composed of two regiments.
Read more about this topic: Royal Tank Regiment