What is it called when authors trade citations?

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)

I believe this is unethical practice but not "officially" misconduct. There is some ongoing research on this, elsewhere. Some sources call this "citation stacking", while others call it "citation cartels". Mind that these terms were coined for journals while they certainly can be applied to authors.

Such citation schemes are an adaptation of other more obvious citation-boosting strategies, such as self-citations or citations demanded by journal editors and reviewers, which are much easier to spot and expose.

These are often referred to as citation circles, or citation cartels. See here.