A strong signal for double parton (DP) scattering is observed in a 16pb−1 sample of ¯pp→γ/π0+3jets+X data from the CDF experiment at the Fermilab Tevatron. In DP events, two separate hard scatterings take place in a single ¯pp collision. We isolate a large sample of data (∼14000events) of which 53% are found to be DP. The process-independent parameter of double parton scattering, σeff, is obtained without reference to theoretical calculations by comparing observed DP events to events with hard scatterings in separate ¯pp collisions. The result σeff=(14.5±1.7+1.7−2.3)mb represents a significant improvement over previous measurements, and is used to constrain simple models of parton spatial density. The Feynman x dependence of σeff is investigated and none is apparent. Further, no evidence is found for kinematic correlations between the two scatterings in DP events.