The dekopon is a kind of citrus fruit which looks like an orange with a sizable, nipple-like protruberance on the top around the stem area. Invented in Japan in 1972 as a cross between the ponkan and the kiyomi, it is the most delicious citrus fruit I have ever tasted (and believe me, I have tried many). It is very sweet, but not too sweet, with just enough delicious tanginess to remind you that you are eating an actual citrus fruit and not some uber food of the gods. I could eat these things all day!
But anyway, yeah, so the dekopon gets its name from the Japanese word deko, meaning "convex", because the dekopon looks just like the Japanese kanji for "deko" (凸). The "pon" comes from ponkan, the Japanese word for the Chinese honey orange, which is one of the fruits crossbred to create the dekopon.
The term "dekopon" is actually a brand name, used by the company that first invented the fruit, and strictly speaking, only applies to the branded fruits still grown by that company in the Kumamoto area of Kyushu island, but it has become a genericized name for the item, much like "Kleenex" or "Xerox." I've seen several other brand names for the fruit, including "kiyopon" and "dekomaru", and the Japanese wikipedia page insists that the fruit has an official name, which is "shiranui" (不知火, meaning "unknown fire"), but I've never seen that ever used and no matter what the brand name is the supermarket you are at is invariably going to write "dekopon" on the price tag.