What is it called when authors trade citations? Citation is one metric of researchers. That is why some of them lend citation to one another. For example, A writes a paper and cites B's work in it, and then B does the same for A, without the citations being relevant to the works. 

Is there any word for such academic misconduct? I can think of "citation exchange", but I couldn't find any reference about it.
In addition to the terms suggested by Scientist, another relatively common term is citation ring, here ring being used with the meaning (from Merriam-Webster):


  7 a : an exclusive combination of persons for a selfish and often corrupt purpose (as to control a market)
  b : GANG


Here are a few examples of usage of citation ring:


  SAGE Publications busts “peer review and citation ring,” 60 papers
  retracted


(Retraction Watch)


  Some other problems of the method were well outlined by Ulrich Korwitz (pp. 107-110): the problems of self-citation and the citation ring [...]


(T. Roper, "Citation Analysis of the Veterinary Literature: A Tool to Understanding Scientific Communication" in Libraries without Limits: Changing Needs — Changing Roles: Proceedings of the 6th European Conference of Medical and Health Libraries, Utrecht, 22–27 June 1998)