An Alibi

1. (law) The plea or mode of defense under which a person on trial for a crime proves or attempts to prove being in another place when the alleged act was committed. Alibi - Wikipedia
2. An excuse, especially one used to avoid responsibility or blame.

Usage notes

• A very good alibi might be described e.g. as perfect, watertight, airtight, solid or iron-clad.
• Some argue this cannot be used in the non-legal sense of "an explanation or excuse to avoid blame or justify action."