Photosynthesis transfers electrons from water to carbon dioxide molecules. Why is this electron transfer called an oxidation reduction process?

Oxidation is the Loss of Electrons, and Reduction is the Gain of Electrons

(Remember oil rig - Oxidation Is Loss, Reduction Is Gain)

Since water loses an electron, it gets oxidised, and since carbon dioxide gains an electron, it gets reduced.

Since both oxidation and reduction occurs, it is called a Redox reaction