Noun
functional root (plural functional roots)
(mathematics) A function which, when applied a given number of times, equals another given function.
x
2
+
1
{\displaystyle x^{2}+1}
is the functional 2nd root of
x
4
+
2
x
2
+
2
{\displaystyle x^{4}+2x^{2}+2}
.