Believed abstraction can be best defined as an idea or concept that is widely accepted or held to be true. However, antonyms for this term would include concepts that are doubted, unproven, or even disbelieved altogether. Thus, words like myth, legend, fabrication, falsehood, untruth, and lie are some of the antonyms of believed abstraction. These words denote an idea that is not rooted in reality, lacking in evidence, or contradicts universally accepted facts. Therefore, it's essential to fact-check and critically analyze every idea presented as an abstraction to avoid being misled by falsehoods or myths.