General synonyms:
* Explain: To provide a clear and understandable reason for something.
* Defend: To argue in favor of something or someone, often against criticism.
* Support: To provide evidence or reasoning to back up a claim.
* Prove: To demonstrate something to be true.
* Validate: To confirm or make something legally or officially acceptable.
* Rationalize: To provide a logical reason for something, even if it is not the real reason.
More specific synonyms:
* Legitimize: To make something legal or acceptable.
* Sanction: To give official approval to something.
* Exonerate: To clear someone of blame.
* Absolve: To forgive someone for their actions.
* Condone: To accept or tolerate something, even if it is morally wrong.
The best synonym for "justify" will depend on the specific situation.