pho ‧ to ‧ syn ‧ the ‧ sis /ˌfəʊtəʊˈsɪnθəsəs, ˌfəʊtəʊˈsɪnθɪsəs $ ˌfoʊtəˈsɪn-/ BrE AmE noun [uncountable] technical
the production by a green plant of special substances like sugar that it uses as food, caused by the action of sunlight on ↑ chlorophyll (=the green substance in leaves)
—photosynthesize verb [intransitive and transitive]