PM

1. When referring to time, pm or p.m. is short for post meridiem or past midday is the time designated after 11:59 AM noon and before 12:00 AM midnight.
2. When referring to OS/2 or an operating system, PM is short for Presentation Manager.
3. When referring to maintenance, PM is short for Preventive Maintenance.
4. When referring to a waveform or a signal, PM is short for Phase Modulation. PM is a method of encoding and transmitting information in a carrier wave.
5. When referring to fiber-optic, PM is short for Polarization Maintaining and is also known as PMF or Polarization Maintaining Fiber. PM or PMF is a type of optical fiber that maintains a fixed polarization. This helps increase the bandwidth and distance of how far the signal can travel. 
6. In chat PM is short for Private Message or Personal Message and is a message only sent between you and one other user.

Also see: AM, Chat definitions, FM, Operating system definitions, Preventive Maintenance, Presentation Manager, Time, Whisper