Conceptional describes an idea or the process of coming up with that idea. If you and your brother are just brainstorming about what your new business should sell, your enterprise is still in the conceptional stage.
Conceptional is an uncommon, rarely used adjective that's sometimes used synonymously with the much more common and generally preferable conceptual: Both words have to do with thinking and concepts, or ideas, but conceptional is often more focused on the conception — origin, or beginnings — of an idea. The word conceptional is more commonly used in the medical world to reference the age of an embryo during pregnancy, from the time of conception. The Latin root of this word is concipere, "to take in and hold."