What is meant by the notation ||x, y||?

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So I am reading through a paper called "A coupled finite volume solver for the solution of incompressible flows on unstructured grids", and this term showed up:

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The authors of the paper did not define this notation and I cannot find what it means anywhere. Does anyone know what they could mean by this? The mdotf is referring to a mass flow rate by the way. Thanks in advance!