Final answer:
A mutagen is any agent that increases the frequency or extent of mutations. Mutagens can be physical or chemical agents that cause changes to an organism's DNA.
Step-by-step explanation:
Any agent that increases the frequency or extent of mutations is called a mutagen. Mutagens can be physical or chemical agents that cause changes to an organism's DNA, leading to mutations. Examples of physical mutagens include UV radiation and X-rays, while chemical mutagens can include exposure to certain chemicals found in smoke or barbecued foods.