A Chrono Trigger is any object or person who unleashes his, her, or its will upon history and changes the course of time. This is the verbatim explanation given by Lucca at the end of Chrono Cross. In Chrono Trigger, the Trigger was the Time Egg used to revive Crono, and Crono himself was a Chrono Trigger (as he alludes at the end of Chrono Cross). Serge was also a Chrono Trigger, as Lynx and Crono both designate him such.

The Chrono Cross is a Dragonian "Element" capable of wondrous healing or transdimensional effects used to restore the dimensions to one and free Schala from the Time Devourer fusion. It was created by the fusion of two Dragonian artifacts in each dimension.

I guess I'll stick with Chrono "Break", since that's what the United States trademark specified.