This is the 6490th most frequent German word.
einstein
"Einstein" is not a standard German noun; it is a surname, most famously associated with physicist Albert Einstein.
Here, 'Einstein' refers to the proper noun and name of the scientist Albert Einstein.
Albert Einstein war ein berühmter Physiker.
Albert Einstein was a famous physicist.
In this sentence, 'Einstein' is used metaphorically to describe someone as being very intelligent or inventive.
Du bist ja ein richtiger Einstein!
You truly are a real Einstein!
Here, there's a phonetic similarity to 'Einstein', but the words 'ein Stein' just mean 'a stone', unrelated to the name Einstein.