Most people can figure out where the word photosynthesis comes from, but it's still a very interesting origin to cover. Photo- is a prefix meaning "light" present in a lot of scientific words. That's from the Ancient Greek word phos, which derives from the Proto-Indo-European reconstruction bha, which means "to shine" and is present in thousands of other words. Synthesis is also something we should recognize: it exists today as a word meaning "to make", and that's exactly its function in this word. Through Latin, it came from Ancient Greek, where it had the same spelling and definition. Now we can eliminate the prefix syn-, which meant "together" or "with" (traces to Mycenaean Greek), which leaves the verb tithenai, "to place" (so synthesis is placing together), ultimately from PIE deh, "to put". The etymology of photosynthesis almost suggests that you're placing light together, but as we know it's meant to evoke placing together molecules in the presence of light.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.