(1) Sum of a set of values x1, x2, ..., xn divided by their number, n. It is often denoted by a bar, e.g. x (the word "arithmetic" is frequently omitted; where a "mean" is mentioned, the arithmetic mean is to be understood) .
(2) The first moment (about the origin) of a frequency distribution, f(x) . (DST)
UNESCO/WMO International Glossary of Hydrology (2nd revised ed., 1992)