In 2017 the Department of Justice directed a special counsel to investigate Russian involvement in the 2016 US elections.

Collusion investigation includes exploring possible links between Trump's presidential campaign and the Russian government.

President Trump expressed anger about the investigation.

There is absolutely no collusion.

I didn't make a phone call to Russia.

I have nothing to do with Russia.

Collusion is secret cooperation for a dishonest purpose.

Collusion can lead to criminal activity, even though collusion by itself is not a crime.