it's funny how many different names this formula has:

√ a² + b² + c² + ...

in engineering, this is called the RMS (Root-Mean-Square) and is used for measuring average signal strength.

in statistics, it's called the standard deviation and tells you how much variance there is in a set of samples.

in trigonometry, it's the solution to Pythagoras' theorem and it gives length of the hypothenuse of a right-angled triangle.

in vector maths, it's the magnitude of a vector, also known as the euclidean norm.

in complex maths, it's called the absolute value of a complex number.