To frighten is to make someone feel afraid. Your new look — pink hair, tattoos, and piercings — will probably frighten your grandparents.
If horror movies and haunted houses frighten you, they scare you, and all it might take to frighten your skittish cat is the sound of the vacuum cleaner or coffee grinder. Frighten is the verb form of the noun fright. although originally the verb was also fright. The Old English source is fyrhtu, "fear, dread, trembling, or horrible sight."