What is it called when sellers secretly agree to set production levels or prices?

Collusion:

1. secret agreement for a fraudulent purpose; connivance; conspiracy

2. (Law) a secret agreement between opponents at law in order to obtain a judicial decision for some wrongful or improper purpose [from Latin collūsiō, from collūdere to collude]

collusive adj

From: http://www.thefreedictionary.com/collusion