The phrase "is an element of" is commonly used in mathematics and refers to a relationship between two sets where one set contains at least one member of the other set. Synonyms for this phrase include "belongs to," "is contained in," "is a member of," "is in," and "is included in." These synonyms are interchangeable and can be used to describe the same relationship between sets. It's important to use the appropriate phrase in order to communicate ideas clearly and accurately in mathematical language. By understanding synonyms for the phrase "is an element of," mathematicians can effectively convey mathematical concepts and ideas.