General synonyms:
* Loss
* Failure
* Defeat
* Setback
* Reverse
* Downfall
* Ruin
More specific synonyms:
* Conquest: (used when someone is defeated in battle or war)
* Overthrow: (used when someone is forcefully removed from power)
* Collapse: (used when something suddenly fails or breaks down)
* Submission: (used when someone surrenders or yields)
* Discomfiture: (used when someone is embarrassed or humiliated)
* Rout: (used when someone is completely defeated and forced to flee)
Figurative synonyms:
* Annihilation: (used when something is completely destroyed)
* Demolition: (used when something is completely broken down)
* Crushing: (used when someone is completely overwhelmed)
The best synonym for "defeat" will depend on the specific context in which you are using it.