"Crashen" is the name of an informal, non-canon, out-of-character-only portmanteau nickname for the potential romantic coupling of Caleb "Crash" Waterfront and Carmen Calandra. Invented (and shipped) by NarayanK, it is currently no more than a concept. The name was chosen over the still excellent alternative, "CarCrash", which caused Bumblebee's death.
This prospective pairing is unique because it has the unbridled support of Optimus Prime and all the Autobots, with Megatron and the Decepticons supporting Krashy (Transformers are property of Hasbro). NK's Optimus Prime, after killing Megatron for his allegiance to Krashy, was quoted as saying Crashen was the "Most beautiful unification of all time" and could save the planet. From Krashy.
The Transformers war over Crashen and Krashy is also notable for making the most sense out of anything ever.