Unimaginable is an adjective that describes something that is difficult or impossible to imagine. The antonyms for unimaginable are imaginable, conceivable, and believable. If something is imaginable, then it is possible to form a mental image or create a mental picture of it. Conversely, if something is conceivable, then it can be thought of or considered. A believable thing is something that is believable or is considered true by most people. Therefore, while unimaginable refers to something beyond the scope of the imagination, imaginable, conceivable, and believable are words that describe things that are within the realm of possibility.