What is monoatomic, diatomic, triatomic and polyatomic ?

Hint: Just break each term in two parts and try to find the meaning of each of them. Think about different gases like oxygen, helium, ozone to get an idea.

Complete step by step answer:
Atomicity is defined as the total number of atoms that constitute a molecule.Now let's find out what is the meaning for each of them:
Monatomic - Monoatomic is a combination of two words “mono” and “atomic” means single atom. This term is applied to the gases as a monatomic gas. In the gaseous phase at sufficiently high temperatures, all the chemical elements are monatomic gases.
e.g. He, Ne, Ar (all noble gases are monatomic)
Diatomic - Diatomic molecules are those molecules that are composed of only two atoms. If a diatomic molecule is composed of the same element it is known as a homonuclear and if it is composed of two different elements it is known as heteronuclear.
e.g. ${ H }_{ 2 }$ , ${ N }_{ 2 }$ , ${ O }_{ 2 }$
Triatomic - The elements that have three atoms are called triatomic elements. They are unstable.
e.g. Ozone ${ (O }_{ 3 })$
Polyatomic - The elements that have more than two atoms bonded by a covalent bond are referred to as polyatomic elements.
e.g. ${ P }_{ 4 }$ , ${ S }_{ 8 }$ , ${ { ClO }_{ 2 } }^{ - }$

Note: All metals and some other elements (such as carbon), do not have a simple structure but consist of a very large number of atoms bonded together. Their atomicity cannot be determined and is usually considered as 1.
Atomicity may vary in different allotropes of the same element.