What exactly is collusion?

 The definition is "secret agreement or cooperation especially for an illegal or deceitful purpose: "acting in collusion with the enemy."  John Brennan, Obama's head of CIA, is a good example.

Working with British and Estonian spies, "John Brennan’s CIA operated like a branch office of the Hillary campaign, leaking out mentions of this bogus investigation to the press in the hopes of inflicting maximum political damage on Trump. An official in the intelligence community tells TAS that Brennan’s retinue of political radicals didn’t even bother to hide their activism, decorating offices with “Hillary for president cups” and other campaign paraphernalia.

A supporter of the American Communist Party at the height of the Cold War, Brennan brought into the CIA a raft of subversives and gave them plum positions from which to gather and leak political espionage on Trump. He bastardized standards so that these left-wing activists could burrow in and take career positions. Under the patina of that phony professionalism, they could then present their politicized judgments as “non-partisan.” "  From The Spectator, April 17, using The Guardian, April 13.

http://www.washingtonexaminer.com/john-brennan-hostility-between-putin-clintons-drove-russias-actions/article/2623908

http://www.americanthinker.com/articles/2017/05/did_brennan_collude_with_foreign_spies_to_help_hillary.html