The cheetah has a thin elongated body and is said to be the fastest runner on earth.

in the above sentence, runner is a noun or not

i think cheetah is the noun.

My question is 'runner' belongs to which parts of speech.

Runner is also a noun.

Sra

Adding -er turns run from a verb into a noun, a thing that runs. Adding -ing would also turn run into a noun, the thing you do when you run. If it is a person, place or thing it is a noun. If you can replace it with the word 'thing'. It is a noun. ...the fastest thing on earth...

In the above sentence, the word "runner" is indeed a noun. It refers to a person or animal that runs. To determine whether a word is a noun, you can consider its function and meaning within the sentence. In this case, "runner" is being described as the fastest, indicating that it is the subject of the comparison being made.