The upper mandible is a term most commonly used in ornithology to describe the upper part of a bird's beak. However, there are a few synonyms one can use instead of upper mandible. 'Upper bill' is a common alternative that is less technical and easier to understand. 'Maxilla' is also a term that can be used interchangeably with the upper mandible, although it is more commonly used in anatomy to refer to the upper jaw of a vertebrate. Lastly, 'rostrum' is another term that can be used for the upper mandible, which is often used to describe the beak of fish or cetaceans. Overall, there are several options for synonymous terms to use for the upper mandible, depending on the context of use.