A modifier is a word that describes or changes another word. Adjectives and adverbs are fun modifiers. They can turn a “man” into a “strange man” or make him “act strangely.”
To modify is to change slightly, so a modifier is anything that makes this change. Modifiers are popular in grammar land, but you could say that chocolate is a mood modifier, for example. An editor, who alters a few sentences, can be described as a modifier of the original manuscript. In the world of science, a modifier is a kind of gene that changes the effect of another gene. Modifier comes from the Latin word modificare, "to limit or restrain."