Element

Category:Terminology
Each element has a weakness and a strength. 
Fire (Element) > Ice (Element) > Wind (Element) > Earth (Element) > Lightning (Element) > Water (Element) > Fire (Element) ... and keeps going.
Additionally Dark (Element) <--> Light (Element) are opposed elements.
Or, if you like lists:
:Fire (Element) melts Ice (Element)
:Ice (Element) compacts Wind (Element)
:Wind (Element) erodes Earth (Element)
:Earth (Element) grounds Lightning (Element)
:Lightning (Element) electrifies Water (Element)
:Water (Element) douses Fire (Element)
:Light (Element) illuminates Dark (Element) 
:Dark (Element) eclipses Light (Element)
Certain weapon skills have elemental properties, as well.
Ex: Red Lotus Blade > Fire
See also:
Weather
Ninja: Elemental Wheel