Washington is obsessed with the word “collusion” but has little understanding of its true meaning. The confusion might explain why D.C. has missed the big story of collusion between Fusion GPS and the Democratic Party.

To read the headlines, a poor, beleaguered opposition-research firm was humiliated and constitutionally abused this week by partisan Republicans on the House Intelligence Committee. Fusion’s lawyers sent a 17-page letter to the committee’s chairman, Rep. Devin Nunes, accusing him of misdeeds, declaring his subpoenas...