1) In research, a measure of the extent to which the subjects undergo an assigned intervention.
Note: It may be the extent to which the subjects comply with the drug therapy, undergo the prescribed medical or surgical procedure, do the prescribed exercise regimen, abstain from smoking, etc.
2) For investigators, the fact of following a research protocol.
3) In a clinical setting, for subjects who are supposed to undergo a treatment or who are registered in a program, the fact of complying with the requirements of that treatment or program.
Note: Since this word has these three meanings, to avoid any confusion, it might be better to refer to “compliance with the research protocol” when what is meant is the investigator’s compliance.