The Collaborative International Dictionary of English v.0.48:
Mean \Mean\, a. [OE. mene, OF. meiien, F. moyen, fr. L. medianus
that is in the middle, fr. medius; akin to E. mid. See
1. Occupying a middle position; middle; being about midway
Being of middle age and a mean stature. --Sir. P.
2. Intermediate in excellence of any kind.
According to the fittest style of lofty, mean, or
3. (Math.) Average; having an intermediate value between two
extremes, or between the several successive values of a
variable quantity during one cycle of variation; as, mean
distance; mean motion; mean solar day.
Mean distance (of a planet from the sun) (Astron.), the
average of the distances throughout one revolution of the
planet, equivalent to the semi-major axis of the orbit.
Mean error (Math. Phys.), the average error of a number of
observations found by taking the mean value of the
positive and negative errors without regard to sign.
Mean-square error, or Error of the mean square (Math.
Phys.), the error the square of which is the mean of the
squares of all the errors; -- called also, mean square
deviation, mean error.
Mean line. (Crystallog.) Same as Bisectrix.
Mean noon, noon as determined by mean time.
Mean proportional (between two numbers) (Math.), the square
root of their product.
Mean sun, a fictitious sun supposed to move uniformly in
the equator so as to be on the meridian each day at mean
Mean time, time as measured by an equable motion, as of a
perfect clock, or as reckoned on the supposition that all
the days of the year are of a mean or uniform length, in
contradistinction from apparent time, or that actually
indicated by the sun, and from sidereal time, or that
measured by the stars.