>("su" is the "superuser" command used to "become" a "superuser", ie. the most powerful user on a UNIX system, where one can perform administrative tasks that o

>("su" is the "superuser" command used to "become" a "superuser", ie. the most powerful user on a UNIX system, where one can perform administrative tasks that ordinary users are not capable of doing) su is "substitute user," but it defaults to substituting the root user.