A Crossbite is a type of malocclusion where some of your teeth are abnormally in front of, or behind, other teeth when you bite down. In the photo above, notice that the teeth on the top left are behind the ones on the bottom left. The teeth on the top right are in front of the ones on the bottom right. This disparity in bite is the Crossbite.
Crossbites are usually corrected with an expander, or other orthodontic appliances.