PT (idea) by Albert Herring

In British English, old-fashioned educational abbreviation for Physical Training; now more usually PE for Physical Education, or just "games". Sekicho informs me that PT in this sense is also used in the US military.
The (now obsolete) US Navy pennant number prefix for a Patrol Torpedo boat (equivalent of a Motor Torpedo Boat, MTB, in other navies). These small craft were not named and were just known by their pennant/hull numbers, the most well known being PT-109 in which John F. Kennedy served.
The international civil aircraft marking code for Brazil.
The ISO two-letter country code for Portugal (PT) and the ISO language code for Portuguese (pt: pt_PT for European Portuguese, pt_BR for Brazilian Portuguese)
The top level domain for Portugal, .pt, which is controlled by the Fundação para a Computação Científica Nacional. Registration is subject to restrictions on the basis of nationality or jurisdiction and type of entity (with tight limits on the actual names used) for all except the .com.pt subdomain.
Common contractions for point, pint, port, part among many others.

Thanks to spiregrain for pointing out an obvious omission...

PT Boat	Scientology's reaction to the WTC attack	How New Orleans stopped Hitler	Battery fungus
People who use Windows	Tazorac	Ptosis	PE
Font size in pixels	Henry Miller	PT-109	Everything Quests - The High Seas
US Navy Jargon - a Selective Glossary	OCS	ROTC	PTO shaft
Perforated	physical chemistry	AU	point
paper towel	Scientology	Starting position