Yes, PIS is a Scrabble word!

Letter values in Scrabble US: P 3, I 1, S 1.

Is it a valid word?

PIS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

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

Also: private detective, private eye, private investigator, principal investigator, protease inhibitor.