An element is any of the four tiles that match up with the four boots: fire, water, force floor or ice. Elements are immovable and only water can be removed from the game at any time, other than uses of advanced coding or the Non-Existence Glitch. The four boots allow normal passage through their corresponding element tile, but force floors and ice may be quicker to pass without boots.
The official introduction to the four elements in harmony is Elementary.
Often, the term is used to simplify explanations that use all four tiles at once.