Der or DER may refer to:
The derivative of a function of a real variable measures the sensitivity to change of the function value with respect to a change in its argument (input value).
Derry (/ˈdɛri/), officially Londonderry (/ˈlʌndənˌdɛri/), is the second-largest city in Northern Ireland and the fourth-largest city on the island of Ireland.
Derby (/ˈdɑːrbi/ DAR-bee) is a city and unitary authority area in Derbyshire, England. It lies on the banks of the River Derwent in the south of Derbyshire, of which it was traditionally the county town.