This morning, Google researchers officially made computing history. Or not, depending on whom you ask.

The tech giant announced it had reached a long-anticipated milestone known as “quantum supremacy” — a watershed moment in which a quantum computer executes a calculation that no ordinary computer can match. In a new paper in Nature, Google described just such a feat performed on their state-of-the-art quantum machine, code named “Sycamore.” While quantum computers are not yet at a point where they can do useful things, this result demonstrates that they have an inherent advantage over ordinary computers for some tasks.

Yet in an eleventh-hour objection, Google’s chief quantum-computing rival asserted that the quantum supremacy threshold has not yet been crossed. In a paper posted online Monday, IBM provided evidence that the world’s most powerful supercomputer can nearly keep pace with Google’s new quantum machine. As a result, IBM argued that Google’s claim should be received “with a large dose of skepticism.”

So why all the confusion? Aren’t major milestones supposed to be big, unambiguous achievements? The episode reminds us that not all scientific revolutions arrive as a thunderclap — and that quantum supremacy in particular involves more nuance than fits in a headline.

Quantum computers have been under development for decades. While ordinary, or classical, computers perform calculations using bits — strings of 1s and 0s — quantum computers encode information using quantum bits, or qubits, that behave according to the strange rules of quantum mechanics. Quantum computers aim to harness those features to rapidly perform calculations far beyond the capacity of any ordinary computer. But for years, quantum computers struggled to match the computing power of a handheld calculator.

In 2012, John Preskill, a theoretical physicist at the California Institute of Technology, coined the phrase “quantum supremacy” to describe the moment when a quantum computer finally surpasses even the best supercomputer. The term caught on, but experts came to hold different ideas about what it means.

And that’s how you end up in a situation where Google says it has achieved quantum supremacy, but IBM says it hasn’t.

Before explaining what quantum supremacy means, it’s worth clarifying what it doesn’t mean: the moment a quantum computer performs a calculation that’s impossible for a classical computer. This is because a classical computer can, in fact, perform any calculation a quantum computer can perform — eventually.

“Given enough time … classical computers and quantum computers can solve the same problems,” said Thomas Wong of Creighton University.

Instead, most experts interpret quantum supremacy to mean the moment a quantum computer performs a calculation that, for all practical purposes, a classical computer can’t match. This is the crux of the disagreement between Google and IBM, because “practical” is a fuzzy concept.

In their Nature paper, Google claims that their Sycamore processor took 200 seconds to perform a calculation that the world’s best supercomputer — which happens to be IBM’s Summit machine — would need 10,000 years to match. That’s not a practical time frame. But IBM now argues that Summit, which fills an area the size of two basketball courts at the Oak Ridge National Laboratory in Tennessee, could perform the calculation in 2.5 days.

Google stands by their 10,000 year estimate, though several computer experts interviewed for this article said IBM is probably right on that point. “IBM’s claim looks plausible to me,” emailed Scott Aaronson of the University of Texas, Austin.

So assuming they’re right, is 2.5 days a practical amount of time? Maybe it is for some tasks, but certainly not for others. For that reason, when computer scientists talk about quantum supremacy, they usually have a more precise idea in mind.