Chronometric radiation is a form of radiation indicative of alterations in the timeline. As the universe attempted to stabilize, the levels of radiation would shrink, approaching zero.
Both the USS Enterprise-D and Goddard noted fluctuating levels of chronometric radiation while they were in an alternate timeline caused by Montgomery Scott rescuing James T. Kirk before he could be absorbed into the Nexus. The levels were strongest whenever a major change occurred - a ship using a gravity slingshot around the Arhennius star to travel into the past, whenever the Guinans came in contact with each other, or when the Enterprise approached the Nexus to return Kirk. (Star Trek novel: Engines of Destiny)