bov-bail out valve. a mouthpiece that has a second stage built in so you can bail out without removing the mouthpiece from your mouth
mav-manual add valve. used to manually add oxygen or diluent to the loop. Historically the same as a drysuit inflator valve on a front mounted counterlung, but have since gotten a bit more diverse
END-equivalent narcotic depth. not a rebreather term but a trimix term. Used to figure out how narcotic and by coincidence how dense a gas is going to be. Same formula as calculating EAD and used to compare how narcotic a gas will be compared to air. I.e. 10/70 at x depth is the same ppN2 as air or EAN32 at y depth.
EAD-equivalent air depth. This is a nitrox term to compare a higher ppO2's gas to air for decompression purposes.
OTS cl's-over the shoulder counterlungs. Google it for a picture, Megalodon is probably the best one to look for but most rebreather manufacturers that don't have integral counterlungs have them as an option. Meg, HH, O2ptima, Liberty, APD, etc.
APD-ambient pressure diving. Making of the Inspiration rebreather, aka YBOD/yellow box of death
C/L-counterlung. flexible bag used to "store" a breath when you exhale into the loop