Simple Definition:

A root word is the basic form of a word, which cannot be broken down into smaller parts and still have meaning. It is the main part of a word from which other words are derived. A root word can be combined with prefixes and suffixes to create new words. For example, the root word "play" can be combined with the prefix "re-" to make the word "replay" (to play again) and with the suffix "-er" to make the word "player" (someone who plays).

Very Simple Definition:

A root word is the most important part of a word that cannot be made smaller and still make sense. It can be used to make new words by adding small parts at the beginning or end. For example, the word "play" can make the words "replay" (play again) and "player" (someone who plays).