RegionMember

RegionMember[reg,{x,y,…}] gives True if the numeric point {x,y,…} is a member of the constant region reg and False otherwise.

RegionMember[reg,{x,y,…}] gives conditions for the point {x,y,…} to be a member of reg.

RegionMember[reg] returns a RegionMemberFunction[…] that can be applied repeatedly to different points. 

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