What is a single term algebraic expression is called?

An algebraic expression which consists of one non-zero term only is called a monomial. Examples of monomials: a is a monomial in one variable a.

Expression with one term is called a 'Monomial'

Expression with two unlike terms is called a 'Binomial'.
Expression with three unlike terms is called a 'Trinomial'.

(6,0) i hope it makes enough sense