A Crossbite is when your top teeth and your bottom teeth do not come together or bite in the correct position.
When that happens it's called a Crossbite.
When you close your mouth and some of your upper teeth are sitting inside your lower teeth rather than on the outside, like they normally would that would mean you have a Crossbite.
Crossbites are also a malocclusion (bite problem).
Dental treatment for a crossbite to fix the crossbite usually involves the use of a palatal expander, a fixed or removable orthodontic appliance used to make the upper jaw wider.
It would be used alongside an appliance designed to move the teeth, such as braces or clear aligners.