A symbol that represents an idea or a thing, rather than the sounds of a word, is called an ideograph. A smiley face emoji is an ideograph that represents happiness.
Many street signs are ideographs, meant to convey a specific meaning without using any words. "Pedestrian crossing" might be a graphic representation of people walking, and "No smoking" is usually a picture of a cigarette inside a circle with a line through it. Hieroglyphics and cuneiform are mainly ideographs, and so are mathematical symbols and emoji. Ideograph combines the Latin idea with graph, "instrument for writing."