What is a 'classic paper', and when do we refer to a work as a 'classic work'? I have come across a few times in research papers or in books, where the authors refer a past (mostly an old) research paper as classic paper. For example, in a book Bratko refers the following:


  The procedural meaning of Prolog is based on the resolution principle for mechanical theorem proving introduced by Robinson in his classic paper (1965).
  [1]


In another example, the author mention such a statement in the abstract itself,


  Notes that the stimulation from a classic paper in the heuristics and biases tradition does not come only from the insights provided into processes of judgment and decision making; ... [2]


What is a classic work? Can we say a work as 'classic work', if it cited more number of times? Or, it is a breakthrough work? Then, how do we define a breakthrough work?

[1] Bratko, Ivan. Prolog programming for artificial intelligence. Pearson education, 2001.

[2] Sloman, S. A. (2002). Two systems of reasoning. In T. Gilovich, D. Griffin, &amp; D. Kahneman (Eds.), Heuristics and biases: The psychology of intuitive judgment (pp. 379-396).
This isn't something that is decided based on objective measures - it's a statement of the author's opinion that history has shown the paper to be very important.

If you, in your professional opinion, believe that the paper has had a major influence on an area of research, for a considerable period of time, then you could call it "classic".  If you don't believe that, or you're not sure, or you don't think you have enough experience to judge, then don't use the word.