An egg-like object capable of conjuring miracles.
The C. Trigger is a Time Egg, and details on its mechanics can be found at that entry. It was probably created by Gaspar in Zeal and brought with him to the End of Time. In short, it is a self-contained Gate, allowing its users to travel to a certain point in time. After it is given to Crono's party, it is used atop Death Peak to open a gate to the instant before Crono was obliterated in the Ocean Palace Incident. Time is frozen, and the party replaces Crono with the Clone. The C. Trigger is broken in the process, shattering before opening the Gate to the moment of Crono's demise.
Somewhat misleading statemenets about the Chrono Trigger were added in the localization, suggesting that the Chrono Trigger somehow required the party's spiritual energy for its activation. Gaspar merely meant to encourage the party by telling them to work hard in the face of adversity.
老人「チャンスは一度きりだよ。 ラヴォスによって生じる 時空のゆがみの一番強い場所…… 死の山の山ちょうで使ってみなさい。 ただし成功するかどうかは…… お前さん達の、クロノへの思い しだいだよ。[END] Old Man: There is only one chance. The strongest point of the space-time distortion generated by Lavos…… You must try using it on the peak of Death Mountain. But will it work...…? That will depend on your thoughts towards Crono!
It's also possible that the distortion of Lavos is what allowed the Time Egg to find the moment of the Ocean Palace Incident, at which Lavos was present.
The meaning of the phrase Chrono Trigger is fully divulged. Serge is called a Chrono Trigger by Lynx, as well as the FATE computer, and on Opassa Beach, Crono names him the "new Chrono Trigger." Lucca then reveals that anything releasing its feelings or desires forcefully on history with the capacity to invoke great change is a Chrono Trigger, and that Gaspar once called the Time Egg such a name.
The Time Egg... The ancient sage, Gasper, once called it the Chrono Trigger, you know!? Each of us allows our feelings to be a trigger... Letting them loose changes our world, time, and history!
Used by: Kid
How it is used: Lynx reveals in Radical Dreamers that the Chrono Trigger, if combined with the Frozen Flame, can (according to legend) grant unimaginable powers over time and space to the point that history can be shaped according to the will of the user of these two powers. Kid carries the Chrono Trigger on her throughout the journey, and shatters it during the battle with Lynx, causing a localized temporal distortion. The fact that she has it with her supports the theory well that Radical Dreamers is a dimension in which Crono was not saved by the C. Trigger.