A unit is a Delphi source file(*.pas). It contains interface, class, type and method declarations and implementations. Unlike some other languages (such as C or C++) which have a separate header file for declarations which is 'included' in the code file, Delphi's units contain all required information for both declarations and implementations in a single file.
Each unit is stored in its own file and compiled separately. Compiled units are linked to create an application. Compiled unit file has (*.dcu) extension.
Units are referenced by other units by being listed in that unit's uses clause.
A unit file begins with a unit heading, which is followed by the sections:
The unit must conclude with the word end followed by a period.