Light (known as Lightning in the SNES/PS/PC versions of Chrono Trigger) is the type of magic used by Crono. It was originally known in the Japanese version of the game as Heaven (天 Ten?), but this was changed to Lightning due to Nintendo of America's policy of not having religious-based concepts in their games. For the DS re-release of Chrono Trigger, it was more-accurately translated as Light. The Light element includes lightning-based attacks.
When meeting Spekkio for the first time, he will explain that there are four types of magic: Fire, Water, Light, and Shadow. Crono's party has magic in them, which allows Spekkio to grant them magic. Crono (Light), Marle (Ice), Lucca (Fire), Frog (Water), and Magus (Shadow) can all learn magic, while Robo and Ayla cannot, due to Robo not being a life form and Ayla existing before magic was created.
There are four Techs that Crono can learn that are Light-based.
There is one Tech that Robo can learn that is Light-based.
There is one Tech that Magus knows that is Light-based.
The following Double Techs are Light-based.