A numerical term used to roughly indicate the bullet diameter only.A much more accurate identification method is the use of the actual cartridge name or designation.

diameter of a tube or gun barrel

Once used only to denote the diameter of a watch movement; now often only indicates type (e.g. mens, ladies, automatic)

The unit of measure for the bore of a rifle or pistol.

1 Distance across the bore measured from land to opposite land. In the U.S. and England this measurement is usually given in .00 or .000 of an inch. In Europe and Asia the measurement is generally expressed in millimetres. Commercial designation of a calibre need only be within several thousands of an inch of the true bore diameter. 2 In ballistics it is a unit expressing comparative dimensions, for example with a bullet if the length is three times the diameter it is described as three calibres long.

In simple terms, it means "diameter". In watchmaking, the term refers to the specific layout and shape of a movement and the bridges, and its various components as well as the designer of the movement.

A measure of the diameter of a bullet, pellet or BB Airsoft replicas typically use 6mm Calibre BBs, although models are available that shoot BBs of 8mm Calibre.  Calibres used by real firearms are typically larger than 6mm and air-gun ammunition is typically smaller.

The inside diameter of a tube, or mortar.

The diameter of a projectile, or the bore of a firearm. In rifled arms this measurement is from top of land to top of land across the bore diameter. This is not always quite what it seems, for example a .38 and a .357 calibre bullet are exactly the same diameter (the same bullet in fact). Quite often the name given to a particular gun, such as a .44 magnum, is a label or a name, not an actual measurement of calibre.

Size of a watch movement also to describe the model, style, or shape of a watch movement.

This term indicates a type of movement (men's calibre, automatic calibre, etc). When a calibre number is accompanied by the manufacturer's mark, it denotes its origin. Originally caliber defined the size of a watch movement.

Refers to the dimensions, form and origin of a movement, and by extension the mechanism itself.