Absorbed glass mat
AGM (Absorbed Glass Mat) batteries are a type of lead acid battery where the entire quantity of liquid acid is contained in fibre glass mats that are sandwiched between the lead plates. Typically AGM cells are sealed and have pressure relief valve as a safeguard. Cells with these valves are refered to as VRLA (Valve Regulated Lead Acid). The fibres in the glass mat provide a path for ions to flow between the plates, and these batteries produce very little gas as a result of the improved recombination that results.
See Also: WikiPedia:Absorbed_Glass_Mat