There is no universally accepted definition of this term. John Preskill originally defined the related term “quantum supremacy” as “…computational tasks performable by quantum devices, where one could argue persuasively that no existing (or easily foreseeable) classical device could perform the same task, disregarding whether the task is useful in any other respect." Given this, we might then define quantum advantage as “the ability to execute an algorithm (or family of algorithms) on a quantum computer in a way that yields a resource savings (such as time, energy, or money) compared to the best-known classical methods running on the best existing classical hardware, especially when the algorithm(s) can be made classically intractable by scaling to larger size.” Another definition might be business-centered: “A quantum computer has realized quantum (economic) advantage when a firm solves a business problem using a quantum computer that they couldn’t solve any other way.” A less stringent definition might be “A quantum computer has realized quantum advantage when it solves a problem in significantly less time, money, or energy than any other known methods to solve that problem”. As you can see, there are many ways to define this term.