A monomial expression is one that has only one term. It can be something like 10x or 8. It is not added to anything or multiplied by anything -- nothing else is done to it.
A polynomial expression is one that has more than one term (so of course binomial epxressions have 2 terms and trinomials have 3). It can be something like
a^2 + b^2
Because there you have multiple monomials being added to make a polynomial.
The number of terms of an expression gives the name of the expression.
The expression that has a single terms is called monomial expression.
For instance, y is an expression. Also 6z is an expression, too.
A binomial expression has 2 unlike terms, added or subtracted:
7y + 8z
A trinomial expression has 3 unlike terms, added or subtracted:
15xz + 17 - 9t
A polynomial expression has more unlike terms, added or subtracted:
x^3 + 4yz - 15u + 18w - 21