Broad question. A critical injury could be many things. Life-threatening like a closed head injury or a torn major blood vessel, multiple rib fractures, etc. But a critical injury may not be life threatening, but could threaten a limb or be a major detterent to quality of life - think of a bad hand injury or a spinal cord injury.
Life-threatening. Any injury that is life-threatening is what I consider a "critical injury". I don't know if an exact definition exists. I have served two deployments to afghanistan, and worked many years at one of the busiest trauma centers in america. I dont know how to define critical injuries... But like justice potter stewart wrote in trying to define pornography... "i know it when I see it.".
Life threatening. Any injury that is in any way life threatening is considered a critical injury.