A weighted mean (or weighted average) is like an ordinary mean, but the observations don't contribute equally - more emphasis is placed on some data values than others; they are weighted by a bigger or smaller amount than 1/n.
A weighted mean (or weighted average) is similar to an ordinary mean, but instead of every value contributing 1/n of the observation's value to the average, it contributes some other amount. Generally weights are restricted to be non-negative.
Weights are useful when the precision or variability of observations are not constant.
