The main thing to understand about “collusion” is that it is not a crime.

You are in good company if that makes you wonder why a non-crime is the subject of a lavish investigation led by Robert Mueller and the 16 alpha prosecutors he has hired — four or five times the number the Justice Department usually assigns for the “trial of the century” that seems to pop up every few years.

The word “collusion,” with just the right dark intonation, makes for a sinister storyline. Criminal cases, however, are not media narratives, so pejorative-word games will not get an investigator very …