Because it isn't used nearly as often as the arithmetic mean, geometric mean, or RMS, this mean doesn't have a common name other than just "the generalized mean with $p=1/2$".
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Bumbble Comm
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This is moderately special,
but I don't think it is special enough
to have its own name.
I would call it "1/2-power mean",
with a possible explanation that the
"$r$-power mean" of
$(a_i)_{i=1}^n$
is $ (\frac1{n} \sum_{i=1}^n a_i^{r})^{1/r}
$".
Because it isn't used nearly as often as the arithmetic mean, geometric mean, or RMS, this mean doesn't have a common name other than just "the generalized mean with $p=1/2$".