A keron is an energy particle. It is utilized in Replicators, with each individual block possessing at least two million keron pathways.
Kerons have yet to be discovered by the Tau'ri, but are known to the Asgard. It is unknown if they were discovered by the Asgard prior to the discovery of the Replicators, or if their discovery was known after they had examined the first Replicators. (SG1: "Small Victories")