Underpopulated is a word used to describe an area where the number of people living there is significantly lower than what it could support. There are several synonyms for this word that can be used interchangeably to describe the same situation. Some of these synonyms include sparsely populated, thinly settled, and low-density. You can use any of these words to describe a region, an island, or a city with a low population density. Other synonyms for underpopulated include depopulated, deserted, and abandoned, which can be used to describe an area where people have left in large numbers, leaving the area empty or near-empty.