Norm type in cost function of ANN

Question

I'm reading a tutorial about ANN. They use the following cost function:

As you can see this equation includes a norm. I'm new to the concept of norm.

My question is what kind of norm they use here (There are Absolute-value norm, Euclidean norm, Euclidean norm of a complex number, Taxicab norm, Manhattan norm, p-norm, Maximum norm, infinity norm, uniform norm, supremum norm, Zero norm, etc.)

Answer 1

Reading the article they state that

The two vertical lines represent the $L^2$ norm of the error, or what is known as the sum-of-squares error (SSE)

Otherwise known as the Euclidean norm, Euclidean length, $L^2$ distance, $ℓ^2$ distance, $L^2$ norm, $ℓ^2$ norm, $2$-norm, or square norm.

Also speaking from my experience, most of the time when a norm is specified without a subscript then you can generally assume that it is the Euclidean norm.