No, we do not define imperialism as “the thing the US does.”
I literally quoted you: "imperialism is a global system headed by the US Empire."
Imperialism was first analyzed as a stage in capitalist development when countries like the UK and Germany were greater imperialist powers, in the years leading up to World War I.
WTF are you talking about? Imperialism has existed longer than capitalism! Sure, the British Empire was imperialist, as was the German Empire. But so was France under Napoleon. So was Spain back in the 15th Century. So was motherfucking Rome, two millennia before capitalism was even invented.
And pointedly, so was Russia, going all the way back to the Tsars. See, for example, Russia's 19th century imperialism (along with Britain; I'm perfectly willing to admit that) in the form of "the great game" fighting over central Asia.
The idea that only western capitalists are capable of being imperialist is just fucking ludicrous nonsense.
By the way, I realize that it was a quick edit and you may have missed it, but you didn't respond to my challenge to actually write the words "Russia invaded Ukraine" instead of passively denying the opposite.