* Aircraft: This is the most common meaning. The noun would be airplane or simply plane.
* Flat surface: In geometry, a plane is a flat, two-dimensional surface that extends infinitely in all directions. The noun would be plane.
* Level of existence: In some contexts, "plane" can refer to a level of existence or consciousness. The noun would be plane.
So, the best answer depends on the context. Can you provide more information about what you're referring to?