That has never been the definition of a recession. All recessions are declared by NBER and NBER has always used a large number of metrics in their calculations. You can see the official list of "peak months" and "trough months" here, and you'll note they don't line up cleanly with 2 quarters of negative growth:

https://www.nber.org/research/data/us-business-cycle-expansi...

How did the ancient Greeks and Romans have recessions without NBER's guidance?

The ancient Greeks did not define a recession as two quarters of negative growth— that has never been the definition of a recession, even if you go back several thousand years.

So how do we know they had a recession? If the definition its only a recession if NBER says so you would have to assert that they did not. Have historians have been disseminating Russian disinformation for centuries?

Before 1800, and especially before 1500, the monetary economy was so small, and the barter economy was so large, that these economies did not have recessions as we understand them today. A modern recession, as we've understood them for the last 200 years, can only exist in a society that has largely converted all economic activity to the monetary economy, and thereby made it vulnerable to the money cycle. If you're interested in the emergence and development of the monetary economy the best books ever written on the topic are Fernand Braudel's history:

https://www.amazon.com/Civilization-Capitalism-15th-18th-Cen...

https://www.amazon.com/Wheels-Commerce-Civilization-Capitali...

https://www.amazon.com/Perspective-World-Civilization-Capita...

I strongly recommend these books.

If you're interest is more about the classical Western period, I recommend:

https://www.amazon.com/Ancient-Economy-Sather-Classical-Lect...

We have had 2 quarters of negative growth before without an official NBER recession (though not since the 50s).

Until recently, a recession has meant two quarters of negative growth.

The term is used to describe economic conditions in various countries.

The NBER is not part of the designation.

Indeed, it would be impossible for an economist to observe that Egypt, for instance, was in a recession as the NBER issues no opinions on the Egyptian economy.

No, conditioning the definition of a recession on the NBER was a silly political stunt by the current admin. Don’t go along with these redefinitions… it’s silly and illogical.

As someone with an MA in econ, most of what you've said here is false. While two quarters of negative growth is often used as an informal measure of recession, the NBER definition is the one that has been used in the US for quite some time by economists.

Historically, here in the US we haven't used the Egyptian definition of recession when talking about the US economy, so what you think about their definition is irrelevant.

That isn't to say that we're not currently in a recession, but economists don't know yet, and neither do you.