A statistical term used as a benchmark in clinical trials. Overall survival refers to the total survival, usually measured from the entry of the patient into the study (or from randomisation) until death (for any reason).
The median survival (after which time 50% of the patients are still alive), which proportion of the patients is still alive after a certain time (e.g. 3 months, 12 months, 5 years, etc.) and is usually considered the "hardest" (least subjectively influenceable) endpoint. Overall Survival is often required by regulatory agencies.
Not very meaningful for the extent of palliation in advanced tumor disease. Therefore, the OS value in palliative situation is less important for clinicians than other parameters (such as PFS or TTF). OS = "gold standard" for assessing the efficacy of adjuvant therapies.