Formal:
* Grievance: This emphasizes a serious cause for complaint, often regarding unfair treatment.
* Objection: This implies a strong disagreement or disapproval.
* Protest: This suggests a formal expression of dissatisfaction.
* Appeal: This is often used in a legal or administrative context.
Informal:
* Grumble: A less formal, often mild expression of dissatisfaction.
* Moan: This implies a whining or complaining tone.
* Whine: This implies a more childish or irritating complaint.
* gripe: Similar to a "grumble," but often used in a more informal setting with friends.
Other:
* Criticism: This focuses on pointing out flaws or problems.
* Issue: This can be a neutral way to describe something that is causing concern.
* Problem: This is a general term for something that needs to be addressed.
The best synonym for "complaint" will depend on the specific situation and the tone you want to convey.