Collusion is secret cooperation especially for an illegal or deceitful purpose. Here's what we know:

(1) Trump, his son, his son-in-law and his former campaign boss all tried to keep the emails and meeting secret for 13 months, lying repeatedly in public that no such meeting occurred, lying when seeking security clearances (which is why the FBI only found out from the NYT) and the meeting was orchestrated by a family friend with Russian contacts who suggest that Don Jr. be careful about letting daddy into the email loop so that plausible deniability might be preserved. The secrecy element is iron clad. That is evidence of one key element of collusion.

(2) Whether the secret meeting was for an illegal purpose is one of the subject of the investigation by Mueller.

(3) There is mounting evidence the meeting was for a deceitful purpose: Much of the information leaked by the Russians was false and fake and Trump, his son, his son-in-law and his top aides have spent months going to desperate lengths to cover up what happened, from firing Comey to disparaging Mueller, our intelligence agencies and the media which discovered the truth.