Acronyms Commonly Used on True Support

A - affair
AP - affair person

BH - betrayed husband
BOM - betrayed other man
BOW - betrayed other woman
BP - betrayed partner
BS - betrayed spouse
BW - betrayed wife

CC - couples counseling/counselor
CM - committed man (in a committed relationship but not legally married)
CW - committed woman

DD - dear daughter
Dday - discovery day: the day a BS is told or discovers on his/her own about the affair
dM - divorced man
DS - dear son
dW - divorced woman

EA - emotional affair
EMA - extramarital affair
EMR - extramarital relationship

fOM - former OM; anytime you see the lower case "f" in front of one of the acronyms it stands for "former"
FOO - family of origin
fOW - former OW
FWB - friends with benefits

H - husband
HOW - happy other woman

IC - individual counseling/counselor
IMHO - in my humble opinion
IMO - in my opinion

KISA - knight in shining armour

LC - limited or low contact; a decision has been made to limit the contact between the participants in the EMR, usually by the OP due to being dissatisfied with the relationship being an EMR or the MP deciding to work on the marriage
LDR - long-distance relationship: a romantic relationship between two people who live far apart and so are unable to meet on a frequent basis (Oxford Dictionaries)

MC - marriage counseling/counselor
MM - married man
MMtH - married man turned husband
MOM, MOW - married other man, married other woman. These are people who are married and in an affair with someone who is also married.
MW - married woman
MWtW - married woman turned wife
MP - married person

NC - no contact: an extended or permanent break with the purpose being to cease contact in the relationship

OA - online affair
OM - other man
OP - This can have three meanings: other person, original poster (the person who started the thread), or original post (the first post in the thread)
OW - other woman

PA - physical affair
PM - private message

R - relationship
rMP - reformed married person
rOM - reformed other man
rOW - reformed other woman: an OW who compromised her morals through the A, who feels deep remorse for her actions, who swears never again to engage in such things, and who advocates other OWs to abandon their As on principle (explanation courtesy of OWoman)

SG - single guy
sMM - separated married man
sMW - separated married woman
stbxH - soon to be ex husband
stbxW - soon to be ex wife

T/J - threadjack (going off topic in somebody else's or even in your own thread)
TLC - tender loving care
TL;DR - too long; didn't read
TMI - too much information

W - wife

xH - ex husband
xW - ex wife