The word "Dialectical" comes from the idea that two seemingly opposite things can be true at the same time. In the context of therapy, the two opposing forces are Acceptance and Change.
Developed by Dr. Marsha Linehan in the late 1980s, DBT was born out of the discovery that focusing solely on change (as traditional CBT does) often felt invalidating to highly sensitive clients, causing them to drop out of treatment. Conversely, focusing solely on acceptance didn't provide the tools necessary to build a "life worth living." DBT solves this by teaching clients to accept themselves as they are in this moment, while simultaneously working hard to change their behavior.