What is the word root in odynophagia?

The word root in odynophagia is "odyn", which comes from the Greek word "ὀδύνη" (odýnē) meaning "pain".

The suffix "-phagia" comes from the Greek word "φαγεῖν" (phagein) meaning "to eat".

Therefore, odynophagia literally means "painful swallowing".