Element

Element[x,dom] or x∈dom asserts that x is an element of the domain dom. 

Element[x,reg] or x∈reg asserts that x is an element of the region reg.

Element[x1|x2|…,dom] asserts that all the xi are elements of dom. 

Element[patt,dom] asserts that any expression matching the pattern patt is an element of dom. 

Please visit the official Wolfram Language Reference for more details and examples on core symbols.