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). 
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; ... 
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?
 Bratko, Ivan. Prolog programming for artificial intelligence. Pearson education, 2001.
 Sloman, S. A. (2002). Two systems of reasoning. In T. Gilovich, D. Griffin, & D. Kahneman (Eds.), Heuristics and biases: The psychology of intuitive judgment (pp. 379-396).