Final answer:
A root word in word formation is a word that forms the basis of other words.
Step-by-step explanation:
A root word in word formation is a word that forms the basis of other words. It is the simplest form of a word and usually cannot be further divided into smaller meaningful parts. For example, the root word 'play' forms words like 'playing,' 'player,' and 'playful.' The root word provides the basic meaning or concept that is expanded upon by prefixes or suffixes.