C-PTSD stands for Complex Post-traumatic Stress Disorder. It's a chronic and more complex form of PTSD - Post-traumatic Stress Disorder. 

The difference is that people with C-PTSD have often had trauma ongoing for a prolonged period of time and often multiple types of trauma and often (but not always) already from childhood.

This can be surviving multiple sexual assaults as a child, or domestic violence, what connects it to becoming this chronic stress is that it threatens your life in one way or another. Either mentally and/or physically.

I have a clinical C-PTSD diagnosis because of trauma since childhood but also adolescence and adulthood.