Noun

1. A measuring instrument for measuring and indicating a quantity such as the thickness of wire or the amount of rain etc.
2. Accepted or approved instance or example of a quantity or quality against which others are judged or measured or compared
3. The distance between the rails of a railway or between the wheels of a train
4. The thickness of wire
5. Diameter of a tube or gun barrel

Verb

1. Judge tentatively or form an estimate of (quantities or time)
2. Rub to a uniform size
3. Determine the capacity, volume, or contents of by measurement and calculation
4. Measure precisely and against a standard
5. Adapt to a specified measurement
6. Mix in specific proportions