Real Dictionary

mean sun in American English
a hypothetical sun thought of as moving uniformly along the celestial equator at a speed equal to the actual sun's average speed along the ecliptic: hypothesized as a basis for keeping time, because the actual sun does not travel along the ecliptic at a u
an imaginary sun moving uniformly in the celestial equator and taking the same time to make its annual circuit as the true sun does in the ecliptic

mean sun in British English
noun: an imaginary sun moving along the celestial equator at a constant rate and completing its annual course in the same time as the sun takes to move round the ecliptic at a varying rate. It is used in the measurement of mean solar time