means The process by which green plants, algae, and some bacteria use sunlight to convert carbon dioxide and water into sugars, releasing oxygen as a byproduct.

from Built from two Greek pieces: phōs, phōtos, "light," and synthesis, "a putting together" (from syn-, "with," and tithenai, "to place"). So at root it means "assembling with light" — a tidy description of the chemistry. The compound is a relatively modern scientific coinage, stitched together in the 19th century once botanists understood that plants were quite literally building themselves out of light and air.