General Antonyms:
* Deterioration: This is the most common antonym, meaning a decline in quality, condition, or value.
* Regression: This implies a return to a worse or earlier state.
* Decline: This is a more general term for a decrease in quality or value.
* Worsening: This means becoming worse.
Specific Antonyms (depending on context):
* Disimprovement: This specifically refers to a lack of improvement or a worsening of something.
* Stagnation: This means a lack of progress or development.
* Degradation: This refers to a decrease in quality or value due to wear, abuse, or neglect.
* Diminishment: This refers to a decrease in size, extent, or importance.
The best antonym for "improvement" will depend on the specific situation and the intended meaning.