the ideal in terms of which something can be judged | a measuring instrument for measuring and indicating a quantity such as the thickness of wire or the amount of rain etc. |
a basis for comparison; a reference point against which other things can be evaluated | the distance between the rails of a railway or between the wheels of a train |
| the thickness of wire |
| diameter of a tube or gun barrel |
| accepted or approved instance or example of a quantity or quality against which others are judged or measured or compared |