BT: Boil Tank/Tun. I have only seen this in discussions of brewing systems. In common discussion, I usually see “boil kettle”, “boil pot”, or just “pot” or “kettle”.
DME: Dry Malt Extract. Sometimes called “Spraymalt” in reference to the process of making DME.
HERMS: Heat Exchanged Recirculating Mash System. A type of RIMS (see below) brewing setup where the mash liquid is circulated through a heat exchanger (in this sense, a fancy word for a copper coil submerged in hot water) in order to maintain the wort at a specific temperature or raise it to a higher temperature. Yes, you could reduce temperature, too, assuming the water around the coil was cooler than the wort, but that’s not our goal in mashing.
HLT: Hot Liquor Tank. In an all-grain system, the HLT is the where hot water is made that is used for mashing and sparging. In a HERMS system (see above) the HLT is usually where the heat exchange coil is submerged.
Lauter: We’ll go with wikipedia on this one: “Lautering is a process in brewing beer in which the mash is separated into the clear liquid wort and the residual grain. Lautering usually consists of 3 steps: mashout, recirculation, and sparging.”
LHBS: Local Home Brew Shop. E.g. “Get some hops at your LHBS.”
LME: Liquid Malt Extract
Mash(ing): The process by which the non-fermentable sugars (starch) in grains are converted to fermentable sugars (mostly maltose, we hope). Basically, you soak some barley, rye, wheat, etc. in some amount of sufficiently hot water for a sufficiently long enough time and you get [sweet] wort, the stuff you ferment to make beer. It is obviously more complicated than that, but that’s the really short version.
MLT: Mash-Lauter Tun. Sometimes referred to as MT (Mash Tun). If you’re doing some all-grain brewing, you’ll need somewhere to mash grains – that’s the MLT. I have occasionally seen “Tank” instead of “Tun”. Semantics, mostly. If we wanted to be specific, we could say MLC, Mash-Lauter Cooler, since many home brewers use coolers of various sizes for mashing and lautering.
RDWHAHB: Relax, Don’t Worry, Have A Home Brew. I usually see it used to tell brewers that they’re worrying too much about what’s going on with a fermenting batch of beer. Even when things go “wrong”, home brew can turn out pretty good, and it’s usually best to enjoy the fruits of one’s labor, even if they’re not precisely what one intended.
RIMS: Recirculating Infusion Mash System. A brewing system where the wort is passed over a heat element of some kind to maintain or raise temperature. A HERMS system (see above) is basically a type of RIMS system, except that the wort is passed through the heat source (hot water) instead of over some kind of heat element.
Sparge: Literally, to “sprinkle or spray.” For brewing purposes, it’s rinsing mashed grains with sufficiently hot water to get all those tasty sugars that yeast love away from the grain and into our boil kettle. Again, there is subtlety here, but that’s the spirit of it.
Wort: Essentially, proto-beer. Technically, it is the sugary liquid extracted from mashing that you boil and add hops (and sometimes other stuff) to produce the liquid that gets fermented by yeast into beer. Ok, so the yeast really only ferment the sugars in the wort, but you get the idea.