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A letter x with a horizontal bar above it, standing for the average of a sample.
Also known as: x bar, Sample average symbol
x̄ — read 'x bar' — is the arithmetic mean of a sample: add the values, divide by how many there are. The bar is what distinguishes it from the population mean μ, and the distinction matters because one is measured and the other is usually unknown.
Denotes the arithmetic mean of a sample: the sum of the observed values divided by their count.
Contexts: Mathematics
Distinguished from the population mean, written with the Greek letter mu. The sample mean is calculated from data in hand; the population mean is a property of the whole population and is usually estimated rather than known.
An overbar denotes an average generally, not only of x — ȳ, r̄ and others follow the same convention.
A lower-case letter x with a single horizontal bar drawn across the space above it.
Both are summary statistics computed from the same sample.
x̄ is the mean of a sample you measured; μ is the mean of the whole population, which you usually cannot measure. Statistics is largely the business of using the first to say something about the second.