Neo- (prefix): Prefix meaning new. From the Greek "neos", new, young, fresh, recent.
Examples of terms starting with "neo-" include neonatal and neonate (the newborn), neoplasia and neoplasm (new growth = tumor), etc.
The opposite of neo- is paleo-. Although paleo- does enter occasionally into medical usage, it is far less commonly encountered than neo- in medicine (and on the pharmacy shelves where there is Neomycin but no Paleomycin).