The Europa Orbiter was a planned NASA mission to Jupiter's Moon Europa, that was cancelled in 2002. Its main objectives included determining the presence or absence of a subsurface ocean and identifying candidate sites for future lander misions.
The results of the studies on the Europa Orbiter have been conducive to the Jupiter Europa Orbiter, NASA's contribution to the planned international Europa Jupiter System Mission (EJSM) that was slated for launch in 2020. EJSM was cancelled in 2011.
The Europa Orbiter-concept should not be confused with the Jovian Europa Orbiter, a feasibility study conducted by the European Space Agency which was finally superseded by the EJSM, too.