pi

(noun) the ratio of the circumference to the diameter of a circle; approximately equal to 3.14159265358979323846....

(noun) someone who can be employed as a detective to collect information. Synonyms: private detective, PI, private eye, private investigator, operative, shamus, sherlock.

(noun) the scientist in charge of an experiment or research project. Synonyms: principal investigator, PI.

(noun) the 16th letter of the Greek alphabet.

(noun) an antiviral drug used against HIV; interrupts HIV replication by binding and blocking HIV protease; often used in combination with other drugs. Synonyms: protease inhibitor, PI.