The Chrono Trigger, I believe, is any one thing or being of such monumental importance to the time-space continuum that it possesses an unnatural power to affect time.
In the original game, any number of such things could be assumed as 'Triggers,' but foremost are two: Crono and Marle's pendant.
Crono seems to be a normal boy. But, for some reason, he was gifted with the bravery to follow Marle through that Gate, thus beginning the whole of Chrono Trigger. Whether that's through fate, manipulation by an Entity, or just pure coincidence that he was there at the time, we may never know. His bravery is potent throughout the adventure, be it the first Gate's formation; the decision to destroy Lavos; or his sacrifice at the Undersea Palace. He's an extraordinary young man, with extraordinary natural talent and courage that makes him worthy of the title "Chrono Trigger."
Second would be the pendant. This is an object so singularly tied to the mysterious Red Rock, bathed in the black light of Lavos' Mammon Machine, that it seems to 'trigger' the Entity's will... Or something along those lines. What my point is, is that the pendant has been irrevocably tied to the fate of the Earth, through its heritage and exposure to Lavos... and, of course, Schala and her family. That pendant, as well as Crono, could be considered a "Chrono Trigger."
Essentially: A Chrono Trigger is any object, place, or person so powerful and essential to the flow of time that it possesses unnatural influence.
... Of course, take all that for what it's worth.
Edit: As usual, I've completely forgotten something. In this case, it was the entirety of Chrono Cross... Dear me, that's frightening.
In Cross, the idea of a "Chrono Trigger" remains mainly the same -- something with an innate, unnatural influence on the timestream that is capable of changing the very fabric of reality and time itself.
We have Serge, of course, as a major CT... Forgive me if I mess anything up, it's been so long since I played Cross. Serge was the focal point of Schala's remaining good, the "trigger" for the splitting of the dimensions through his connection to the Flame and Lynx's attempted (or, actually completed) drowning. In his position as Arbiter to the Frozen Flame -- and, therefore, Lavos/TD -- he is an integral part of all space-time's fate. This is a supernatural kid we have here -- nearly died twice, came into contact with an otherworldly object, destined to decide the fate of all space-time... He's got a lot on his shoulders. Why his cries, of all people's, was heard by Schala, I cannot say... Perhaps the Planet imbued him with some special connection to her and the DBT at birth, and at random, as an instrument of destroying the Time Devourer. Or, maybe Belthasar somehow affected things. What matters to me, is that he has almost absurdly bad luck to be chosen to endure all that and more.
We also have the Time Eggs, namely Kid's amulet and Belthasar's Egg. These are created by, arguably, two of the greatest geniuses in history -- Lucca and Belthasar, respectively -- and are so well-manufactured, brimming with such power, that they can serve as Chrono Triggers in a separate, but similar way to Serge. I honestly don't remember much about them, so I will refrain from further comment and speculation.
And then there's Kid, a clone of Schala, forged from the Darkness Beyond Time... Who is inevitably tied to Schala and her binding, as well as carrying the Astral Amulet. As she's so undeniably tied to Schala, I'm thinking she is a Chrono Trigger in the same vein as Serge -- affected by a force from the Darkness Beyond Time -- but in an even stronger sense, as she was actually birthed by Schala in Lavos' captivity.
... I honestly don't know what else to say. Discount my ramblings if you wish... It's been far too long since I've played the actual games to really add insightful commentary. And, I haven't really read any of the articles here in a LONG time, either...
Sorry for intruding.
