This will be a managed thread. PM me with your question or contribution. I will update this post and then remove your post, just to keep this clean. I want to keep it in alphabetical order. I'm starting with the post that Jackalope had.

BSF - Black Soldier Fly

BOD – Biological oxygen demand

BP – Barrelponics

CF – Compact fluoro

CF – Continuous Flow

CHIFT PIST - Constant Height In Fish Tank, Pump In Sump Tank

CHOP - Constant Height One Pump

DFT - Deep Flow Technique

DI - Deionisation

DO – Dissolved oxygen

DOC – Dissolved organic compounds

DWC – Deep water culture

E&F – Ebb and flow

F&D – Flood and drain

FRC – Floating raft culture

FT – fish tank

GAC - Granulated Activated Carbon/Charcoal

GB – Grow beds

GH – General Hardness, caused by divalent metal ions, primarily calcium and magnesium

GH – Green house

HPS – High pressure sodium lighting

IBC – Industrial / International / Intermediate bulk container

KH – Carbonate Hardness, due to carbonate / bicarbonate ions, and represents the main 'buffering capacity' of the water, i.e. its ability to resist pH changes

LC50 – Lethal Concentration 50 usually used for gases in air or in the case of fish concentration in water in ppm. The 50 represents a 50% mortality in test subjects.

LD50 – Lethal Dose 50 at which 50% of the test subjects died. Is usually stated in mg/kg body weight.

Meg/L - Millequivalents per litre

MH – Metal Halide lighting

NC - Normally Closed (refers to relay contacts) contacts closed when relay has no power applied to relay coil (eg 240v), opens when power on

NFT - Nutrient Film Technique

NH3 – Ammonia

NH4 – Ammonium, the ionized form of ammonia

NO - Normallly Open (refers to relay contacts) contacts open when relay has no power.

NO2 – Nitrite

NO3 – Nitrate

PAC - Powdered Activated Carbon/Charcoal

PC - Power Compact Lights

pH - acidity or alkalinity of a substance

ppm – Parts per million

ppt - Parts per thousand (i.e. one gram per 1000g or 1Kg per 1000L)

RAS – Recirculating Aquaponics System

RO - Reverse Osmosis

SG - Specific Gravity, meaning the salinity (salt) content of your water.

SHO - Super High Output (refers to lighting)

TBPC - Tri-Based Pelletized Carbon

UGF – Under Gravel Filter

UPS - Uninterruptible Power Supply

UV – Ultra Violet

VHO - Very High Output (refers to lighting)