What are “first class” objects?

Question

When are objects or something else said to be \"first class\" in a given programming language, and why? In what do they differ from languages where they are not?

EDI

Answer 1

From a slide in Structure and Interpretation of Computer Programs, lecture 2A (1986), which in turns quotes Christopher Stracey:

The rights and privileges of first-class citizens:

• To be named by variables.
• To be passed as arguments to procedures.
• To be returned as values of procedures.
• To be incorporated into data structures