X10 Can be ambiguous, sometimes referring to the X-10 company, other times referring to the X10 powerline transmission protocol.
PM Personal Message or Private Message (via the forum message system)
AHP ActiveHome Pro (generally referring to the software but occasionally to the CM15A interface)
AM The AHP Activity Monitor (but sometimes may be used to mean Appliance Module)
SDK Software Development Kit (for AHP; a free download from the X-10 website)
AH The old ActiveHome program which uses the CM11A serial port interface or sometimes the interface itself.
PLC Power Line Code or Power Line Command (generally via the X10 protocol)
LM465, AM486, WS467, etc. Model numbers for various X-10 modules - check the X-10 website if they're not further described
OT Off TopicA few more are:
OP Original Poster