Seaforium is a type of volatile explosive compound in the Warcraft universe that can come in a powder  or a malleable putty. It's most often portrayed as being used in bombs and other such high explosive devices, but has also been used in low explosives, most notably as a powerful (and very dangerous) rocket fuel. According to Ashlan Stonesmirk, "Seaforium is harmless enough, but once it's wet it could blow Ironforge out of the mountain".
In terms of items, seaforium usually refers to the various kinds of Seaforium Charge:
Because of the way it's pronounced, seaforium may be a reference to the real-life explosive compound, C-4.