Sealed lead acid battery
A sealed lead acid battery or gel cell is a lead acid battery that has the sulfuric acid electrolyte coagulated (thickened) so it cannot spill out. They are partially sealed, but have vents in case gases are accidentally released for example by overcharging. They can be used for smaller applications where they are turned up side down. They are more expensive than normal lead acid batteries, but they are also safer.
They come in many sizes, including D, C, AA, AAA, and 6V.