What is considered NOT first-class member?

What is considered NOT first-class member?
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#1

i’m reading this

And there stands
The lexical scope look-up process only applies to first-class identifiers, such as the a, b, and c.

I found googling, that first-class means function literal, and ways to use them like arguments to the function or by returning function as return value…

Question === q title


#3

Why that word need to be introduced?