Imaginable means something that can be conceived or visualized in one's mind. It is possible to think about, and it exists within the realm of possibility.
Usage examples
The devastation caused by the hurricane was beyond imaginable.
She had gone through every imaginable hardship in her life.
The amount of money he donated to the charity was beyond any imaginable sum.
The creativity and talent displayed in their performance was beyond imaginable.
The new smartphone had every imaginable feature that a user could desire.
The luxury resort offered every imaginable amenity to ensure a perfect vacation experience.
The athlete's endurance and strength were beyond imaginable, breaking multiple records in their sport.
The horror movie had the scariest, most blood-curdling scenes imaginable.
Or, if nowhere can be that safe base, they are really in trouble, as being chronically stressed is about the worst thing imaginable for brains, especially developing ones. (scientificamerican.com)