An alias is a false name, not a false description or personation: His real name was John Smith but he ran his business under the alias of Joe Bloggs. An alibi is properly a statement that a person was somewhere else at the time a crime was committed: His alibi was supported by his sister, who had been visiting him that evening. It can also be used of a general excuse: My alibi is that the train was late. However, this is thought unacceptable by some people.