Principia Mathematica, in set theory

Permanent magnet

Pontifex Maximus, The Pope (P·M·)

Prime minister, in government

Particulate matter, fine dust and soot suspended in the air

Precious metal

post mortem

Perry Mason

Saint-Pierre and Miquelon: ISO 3166-1 alpha-2 country code

Promethium, in chemistry (Pm)

Preventive maintenance, aka preventative maintenance, aka proactive maintenance

Post meridiem (also written P.M. and p.m.), Latin for "after noon" - this is used to disambiguate hours of the day, in the 12-hour clock system

Pistolet Makarova

Phillip Morris

Perl module

Pobjoy Mint, via its mint mark "PM"

Palm mute, in guitar tablature

Pere Marquette Railroad (AAR reporting mark PM)

Polar Medal

PM (newspaper), a politically leftist U.S. newspaper in the 1940s

Powdery mildew, a fungal disease that affects a wide range of plants

Phase modulation, in electrical engineering

P.M. (author)

The Peoples of Middle-earth by Christopher Tolkien

Postmaster (or Postmistress) refers to the head of an individual post office. When a postmaster is responsible for an entire mail distribution organization (usually sponsored by a national government), the title of Postmaster General is commonly used....

Picometre, a length unit (10^-12 m) (pm)

Tropic Air IATA airline designator

Personal message or Private message, in the context of BBS's and Internet forums

Private Message in Internet Forums

PM (ABC Radio), an ABC Radio National newsmagazine

Project manager or Project management

PM (Radio 4), a BBC Radio 4 news and current affairs programme