The French word calibre is commonly used to refer to a specific type of watch movement and is often transliterated to caliber in English or abbreviated as Cal. The word derives from the diameter of the movement and this is reflected in the traditional use of the French ligne unit for these metrics. Ebauches were commonly specified by their calibre (diameter) to differentiate various models offered by a manufacturer, though most vendors also had internal reference numbers as well. Today, the word is typically used to specify a movement model and it is rare to encounter an actual diameter measurement.
The use of the term to refer to a type of movement rather than its size dates back to at least the turn of the century, when most watchmakers relied on third-party ebauches for their watches. Given that ebauches were quite simple at this time, and most makers only had a few models, it was natural for watchmakers to specify the size needed rather than the function or other qualities. The 1930 advertisement at right shows the word used both for the type of movement and for its size, with each specified by its diameter. Later movements were specified by size, reference, and function (e.g. the Valjoux Cal. 13”’ Ref. 72 VZH) before standardizing on a simple reference number (e.g. today’s ETA Cal. 2824-2).
Calibre is sometimes assumed to be related to calipers, since the latter is a tool used to measure diameter. But calibre is properly measured by a calibre à limites (go gauge) not a pied à coulisse (calipers) and the terms are unrelated.