Calculus teacher: It reminds you that is the variable in the integration.
Private math. tutor: Points will be deducted if one forgets to write it down.
Engineer: It is an infinitesimally small .
Teacher of advanced calculus: It is the differential of the function .
Measure theorist: We usually write instead.
Differential geometer: It is a smooth function mapping a point in a manifold to an element in the cotangent space at .
Algebraic geometer: Let be a -algebra over a field . is the equivalent class in the quotient , where is the -submodule of generated by elements of the form . In general, is .