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)