Alibi was originally a Latin word. It means 'elsewhere', or 'other-where'. In English, it was used by lawyers, members of one of the 'learned professions', to name a particular defence against a criminal charge in court. This is the defence where the lawyer for the defence states, "My client was somewhere else at the time of the alleged crime - therefore it was not he who committed it." (The accused might say, in less formal grammar, "It wasn't me - I've got witnesses [who saw me somewhere else].")
The word has since often been mis-applied by criminals, who are not often members of learned professions, to mean a defence of any kind, and by members of the general public to mean an excuse of any kind.
In academic English, only use alibi in its original sense - especially if you are a student of Law.