A principal investigator (PI) is the lead scientist or engineer for a particular well-defined science (or other research) project, such as a laboratory study or clinical trial. It is often used as a synonym for `head of the laboratory` or `research group leader`, not just for a particular study. In the context of USA federal funding from agenc...
The person who is designated to take overall responsibility within the team of researchers for the design, conduct and reporting of the study.
The head researcher responsi for organising and overseeing a clinical trial ... (09 Oct 1997) ... 
Principal investigator: In biomedical research, the person who directs a research project or program. The principal investigator (the PI) usually writes and submits the grant application, oversees the scientific and technical aspects of the grant, and has responsibility for the management of the research.
1. The designation used by the National Institutes of Health to denote the individual named on a grant who is responsible for directing the proposed research. 2. The lead scientist in a research project. (Usage note: It is best to avoid use of the term to designate the head of a center in a multicenter trial. It should be used in such settings only...
[n] - the scientist in charge of an experiment or research project
The person who is responsible for the scientific and technical direction of the entire clinical study.
noun the scientist in charge of an experiment or research project
The person who is responsible for the scientific and technical direction of the clinical trial at a specific clinical site. In most cases the principal investigator will be a leading physician in the disease area being studied.