What is a word within word?

You're describing a hidden word or a word within a word. This is a type of word puzzle where a smaller word is concealed within a larger word.

For example:

* "Hidden" is hidden within "Hidden"

* "Rain" is hidden within "Strain"

* "Art" is hidden within "Heart"

There are many other examples, and some puzzles might involve finding multiple hidden words within a single word.