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A datum is something used as a basis for calculating and measuring. In the case of GPS, datums are different calculations for determining longitude and latitude for a given location.
Currently, Geocaching uses the WGS84 datum for all caches. Many maps still use NAD27, which can cause confusion if your GPS unit is set to NAD27. Always check your GPS to ensure that WGS84 is the datum before entering a cache coordinate into your unit