The term "lunar mare sea" is commonly used to refer to the large, dark basaltic plains on the moon's surface which were once believed to be actual seas. However, there are various synonyms and terms that can be used to describe these features. Some alternatives include lava plains, basaltic plains, dark regions, and lowlands. Another set of synonyms are words that describe the characteristic shape of these features, such as craters, rilles, or valleys. While the term "lunar mare sea" remains a popular way to describe these features, the many synonyms available provide a more varied and detailed description of these formations on the moon.