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Definition
Sometimes when people are talking about repeatability they will differentiate between validity and repeatability. The validity of any measurement technique is the extent to which it measures what it purports to measure. A set of bathroom scales report your mass, which is calculated on the basis of the force that gravity exerts on the scales when you stand on them. These should have good validity anywhere you use them since the acceleration due to gravity is 9.81ms-2 anywhere on the surface of the earth. However, if you took them to the moon their validity would be compromised. Your mass would not have changed dramatically but yet the scales would record you as having only 17% of the mass you had when you left earth. The problem here is that the scales were calibrated for one environment and then asked to operate in another, quite different, environment that influences the operation of the scales.
Further reading
An excellent recent review on calculating measures of repeatability is:
Wolak ME, Fairbairn DJ, Paulson YR (2012) Guidelines for estimating repeatability. Methods in Ecology and Evolution, 3, 129-137.