Thermal is a word to describe a dual-pane lens that gives it the anti-fog property. A paintball goggle fogs up when hot, humid air comes in contact with cooler, dryer air, resulting in the condensation we see as fog.  In a double-pane lens, the space between the two panes traps air, and the temperature of this air changes as it draws heat from the inner pane and releases heat to the outer pane. This function balances the temperature differences between all the panes and makes it less likely to fog up.