al·i·bi

/ˈæləˌbaı/ noun, pl -bis [count]

1 : a claim that you cannot be guilty of a crime because you were somewhere else when the crime was committed

Nobody could confirm his alibi that he was at the movies.

He has an ironclad/perfect alibi. [=an alibi that cannot be proved false]

also : evidence which shows that such a claim is true

Her doctor is her alibi: she was in surgery at the time of the murder.

2 : an excuse for not being somewhere or doing something

She made up an alibi for why she missed the meeting.