PI vs principal investigator

PI and principal investigator both are nouns.

Word	Noun	Adjective	Verb	Adverb
PI	Yes	No	No	No
principal investigator	Yes	No	No	No

As nouns, PI and principal investigator are synonyms defined as:

PI and principal investigator: the scientist in charge of an experiment or research project

PI (noun)	principal investigator (noun)
an antiviral drug used against HIV; interrupts HIV replication by binding and blocking HIV protease; often used in combination with other drugs	the scientist in charge of an experiment or research project
someone who can be employed as a detective to collect information and assist in criminal investigations
the scientist in charge of an experiment or research project