ALIBI and EXCUSE
Alibi (noun) means “in or at another place.”
Alibi is usually used in legal contexts (legal situations, such as trials).
It is used both as an adverb and as a noun meaning “a plea of having been elsewhere.”
An alibi is an excuse that is used especially to avoid blame.
For example:
The man who was on trial could not have committed the crime because he had an air-tight (perfect/un-questionable) alibi, he was in a different city at the time of the crime, and there are 10 people who will say (testify) that he was there at that time.
Excuse
An excuse (noun) is an explanation that is offered as a reason for being excused after doing or saying something wrong. It's also a reason why one might be forgiven for making a mistake or as a good reason for not being able to do something that you should have done or promised to do.
- His excuse for being late was accepted.
- He said he was sorry for forgetting her birthday becuase he had too much on his mind at that time, but she did not accept his excuse.
- He uses his poor health as an excuse for not paying attention to all his responsibilities.
Summary:
Alibi - legal term meaning "was not there at that time"
Excuse - a reason for failing to do or say something.