If you're thinking of the book of maps:
* A single map: An atlas is a collection of maps, so a single map could be considered the opposite.
* A globe: While an atlas is a flat representation of the Earth, a globe is a 3D model.
* A guidebook: Instead of showing locations, a guidebook focuses on information about places.
If you're thinking of the mythological figure Atlas:
* A being who doesn't bear the weight of the world: This is more metaphorical, but it contrasts with Atlas's traditional role of holding up the sky.
If you're thinking of the anatomical term for the first cervical vertebra:
* Another vertebra: "Atlas" refers to the first cervical vertebra, so any other vertebra could be considered an opposite.
* A non-vertebral bone: Any bone that isn't part of the spinal column could be considered an opposite.
Ultimately, the best opposite for "atlas" depends on the context.