Answer: ∈ This symbol means "belonging to" a certain "set" in mathematical "set theory". It is the relationship between elements and sets, or between sets. For example, 2∈{ 1, 2,3} indicates that element 2 belongs to this set1,2,3; Another example is: A∈B, which means that set A belongs to set B, that is, A is a part of b (in mathematical language, set A is a subset of set B).