Noun

1. an antiviral drug used against HIV; interrupts HIV replication by binding and blocking HIV protease; often used in combination with other drugs
2. the 16th letter of the Greek alphabet
3. the scientist in charge of an experiment or research project
4. someone who can be employed as a detective to collect information
5. the ratio of the circumference to the diameter of a circle; approximately equal to 3.14159265358979323846...