An outcome is any measure of the patient’s health, such as a score on a pain or disease severity scale, rehospitalization, death, developing a condition, advancement or remission of a cancer, scores on a screening measure, patient-reported quality-of-life, peak volume of oxygen, blood pressure, number of suicide attempts, etc. It can literally be any objective description of the participant’s state of health or the state of their condition, though only certain outcomes will be predetermined to be collected.
Deeper dive
Objective, outcome, and endpoint are often confused, particularly outcome and endpoint, which are sometimes synonyms and other times separated by a very subtle distinction. One thing that is true of all three, however, is that they should be determined before the study begins (a priori) and should not change during the course of the study. If any do change – something a journalist may be able to look up in clinicaltrials.gov or simply ask the researcher – then it’s a good idea to ask the researchers why they changed it and to consider whether it could indicate p-hacking or another statistical red flag.
An objective is the reason for doing the study, what the researchers want to accomplish. The objective could be to compare the efficacy of two interventions, to determine whether one is non-inferior, to determine incidence or prevalence of a condition or whatever else the researchers deem the purpose of the study to be.
An endpoint is the change (or non-change) that has occurred that the researchers are measuring. An endpoint can be, and often is, an outcome, but an outcome is not necessarily always an endpoint. For example, in a vaccine study, the endpoint might be the effectiveness of the vaccine. But vaccine effectiveness does not necessarily comprise participant outcomes. Outcomes would include whether participants did or did not get the disease and any adverse events they may have experienced.
In a study testing a new cardiovascular drug, however, the endpoint and outcome may be the same: the number of cardiac events, such as heart attack, stroke, heart failure, etc.
There are other possible subtle distinctions between endpoints and outcomes, as this crowdsourced answer suggests, but those are not necessarily agreed-upon distinctions.