An abbreviation is a shortened form of a word.

Within abbreviations there are sub-categories:
acronym: a new word created from first letters, and read like a complete word
initialism: a new word created from first letters, but read letter-by-letter

Abbreviations (shortened words)

app

application (as in ‘mobile application’ for phones)
Ms.

Miss
approx

approximately
n.b.

note (nota bene)
blog

web log
no.

number
e.g.

for example (exampli gratia)
op

operation
est.

established
stat

statim (= immediately)
etc.

etcetera (= and so on)
tbsp

tablespoon
frat

fraternity (often used for university fraternities that party, rather than more serious fraternities)
temp

temporary
ID

identification
tsp

teaspoon
i.e.

that is (id est)
vet

veteran
misc.

miscellaneous
vet

veterinarian
Mr.

Mister
vs.
versus
Mrs.

Mistress (however Mrs is pronounced ‘missus’)

A comprehensive list of acronyms and abbreviations can be found at abbreviations.com

Acronyms

aga

Aktiebolaget Gasaccumulator
mooc

massive open online class
AIDS

Acquired Immune Deficiency Syndrome (later stage of HIV)
nimby

not in my back yard
asap

as soon as possible
PIN

personal identification number
AWOL

absent without leave (= they have left, but they shouldn’t have)
radar

radio detection and ranging
BASE jummping

building, antenna, span, Earth
SARS

Severe Acute Respiratory Syndrome
captcha

completely automated public Turing test to tell computers and humans apart
scuba

self-contained underwater breathing apparatus
care package

Cooperative for Assistance and Relief Everywhere (previously Cooperative for American Remittances to Europe; care packages were started during WWII)
SIM card

subscriber identification module
E-fit

electronic facial identification technique
snafu

situation normal, all f**ked up
Gestapo

Geheime Staatspolizei (German; translated: secret state police)
sonar

sound navigation and ranging
GIF

graphics interchange format
Soweto

south west townships
gulag

Glavnoye Upravleniye Ispravitelno-trudovykh Lagerey (Russian; translation: Chief Administration of Corrective Labour Camps)
swag

stuff we all get
humvee

high-mobility multi-purpose wheeled vehicle (HMMWV has been changed to humvee)
taser

Thomas Swift’s electric rifle
laser

light amplification by stimulated emission of radiation
zip code

zone improvement plan

The website Acronym Finder can help you find the meaning to acronyms you don’t recognise or know.

Initialisms

AA

Alcoholics Anonymous (a group that helps people with drinking problems)
GI

galvanised iron (now noted as ‘government/general issue’). GI is an American soldier, and the acronym describes their equipment.
AD

anno Domini (‘in the year of the Lord)
GMT

Greenwich Mean Time
ADD

Attention Deficit Disorder (a problem of not being able to stay still or concentrate for long)
HIV

Human Immunodeficiency Virus
BA

Bachelor of Arts
LGBT+

lesbian, gay, bisexual, transgender, plus others
B & B

bed and breakfast
MA

Master of Arts
BC

Before Christ
MBA

Master of Business Administration
BYOB

bring your own bottle (on party invitations, or at restaurants = bring whatever you want to drink)
MIA

missing in action (= they went to do something, and haven’t come back)
CCTV

closed circuit television
MRI

magnetic resonance imagining
CD

compact disc
NGO

non-governmental organization
CEO

Chief Executive Officer
OR

operating room
CFO

Chief Financial Officer
PA

personal assistant
DIY

do it yourself
PhD

Doctor of Philosophy
DUI

driving under the influence (= drunk driving, or driving whilst on drugs)
p.s.

post script
DVD

digital video disc (or digital versatile disc)
RIP

requiescat in pace (now adapted to ‘rest in peace’)
EMT

emergency medical technician
RnB

rhythm and blues
ER

emergency room
RSVP

repondez s’il vous plait (French; translated: please reply)
ETA

expected time of arrival
UAE
United Arab Emirates
IB

International Baccalaureate
USA

United States of America
IPO

initial public offering
USB

universal serial bus

Company & Organisation Names

BBC

BBC – British Broadcasting Corporation
IKEA

Ingvar Kamprad Elmtaryd Agunnaryd (founder’s name and home)
BMW

Bayerische Motoren Werke (German; translation: Bavarian Motor Works)
IOC

International Olympic Committee (original French: CIO – Comité international olympique
BRIC

Brazil, Russia, India, China
MGM

Metro-Goldwyn-Mayer
CIA

Central Intelligence Agency
NASA

National Aeronautics and Space Administration
CCP

Chinese Communist Party
NATO

North Atlantic Treaty Organization
CNN

Cable News Network
NBA

National Basketball Association
DHL

Dalsey, Lynn, Hillblom (company founders)
NHS

National Health Service
ESPN

Entertainment and Sports Programming Network
NRA

National Rifle Association
EU

European Union
NSA

National Security Agency
FBI

Federal Bureau of Investigation
QANTAS

Queensland and Northern Territories Air Service
FIFA

Fédération Internationale de Football Association (French; English = International Federation of Association Football)
UN

United Nations
GCHQ

Government Communications Headquarters
WHO

World Health Organization
IBM

International Business Machines Corporation
WTO

World Trade Organization

SMS Language

143

I love you (1 letter in ‘I’, 4 letters in ‘love’, 3 letters in ‘you’)
hmu

it me up (i.e. message me)
1432

I love you too (2 = too)
h/t

hat tip (a tip of the hat = thanks or well done)
2day

today
ic

I see
2nite

tonight
iirc

if I remember/recall correctly
4

for
imho

in my honest opinion
afaik

as far as I know
imo

in my opinion
b/c

because
lbs

laughing but serious
brb

be right back
lmao

laughing my ass off (= that was very funny)
cu

see you
lol

laugh out loud (= that was funny)
cu l8r

see you later
np

no problem
da

the
roflmao

rolling on the floor laughing my ass off (= that was hilarious)
dws

dealing with sh*t (i.e. busy)
teh

the
F2F

face-to-face
thx

thanks
ffs

for f***s sake
tl;dr

too long, didn’t read
fwb

friends with benefits (i.e. a friend you have sex with)
ttyl

talk to you later
fya

for your amusement
wtp

what’s the plan?
fyi

for your information
wyd

what you doing?
gr8

great
xx

kisses

A longer list of chat slang can be found here