noun. The definition of an alias is a name different than a person’s birth name. 1  An example of an alias is when a person goes into the Witness Protection Program, and they have to assume a new identity with a new name so no one can find them.lias-Legal Definition. n. 1  An assumed or additional name used by a person, frequently to conceal her true identity, or such a name applied to a person by others. 2  See also known as. 3  An alias writ. 4  Link/Cite.