Depends on the context. A 'research participant', also called a 'human subject' or an experiment, trial, or study participant or subject, is a person who participates in human subject research by being the target of observation by researchers. But a participant does not have to be a subject, or vice versa. "subject" is defined as; a person or thing that is being discussed, described, or dealt with. "participant" is defined as; a person who takes part in something.