Final answer:
An antonym for exertion is idleness, which means a lack of activity or exertion.
Step-by-step explanation:
An antonym for exertion is idleness. In the English language, 'exertion' refers to the physical or mental effort applied towards achieving something. Therefore, the concept of idleness, which implies a lack of activity or exertion, stands in stark contrast. Idleness suggests a state of being inactive or not engaging in any significant physical or mental effort, which can be considered the direct opposite of exertion.