Used for Unit external definitions, and as a Class skeleton System unit
1 keyword Interface(Unit Name;
2 type Name = Interface{(BaseInterface,...)};
The Interface keyword is used in two different ways.
Version 1
It starts the definition of external interface of a Unit. Declarations here are externally visible by other units. All of these declarations must be implemented in the Implementation section.
The Uses statement, if present, must be at the start.
Version 2
In Object Oriented programming, we often use Abstract class methods in a base class as a placeholder. All derived classes must implement these methods.
Taking this one step further, an Interface defines a grouping of just abstract properties and methods. It provides a template for a class to use to ensure consistency. It is like a class with only abstract methods. It has the benefits that classes can be based on one parent class, and implement one or more interfaces. It adds a predictable flavour of operation to each class that implements the interface.
Take a look at the Delphi tutorial for more on this complex subject.
When implementing an interface, you must implement QueryInterface, _AddRef and _Release standard interface methods, unless you base your class on one that already has these implemented, such as TInterfacedObject.

Thanks to Dietmar Brueckmann for the example code.
Related commands
AbstractDefines a class method only implemented in subclasses
ClassStarts the declaration of a type of object class
ConstructorDefines the method used to create an object from a class
DestructorDefines the method used to destroy an object
FunctionDefines a subroutine that returns a value
ImplementationStarts the implementation (code) section of a Unit
ObjectAllows a subroutine data type to refer to an object method
ProcedureDefines a subroutine that does not return a value
TObjectThe base class type that is ancestor to all other classes
UnitDefines the start of a unit file - a Delphi module
UsesDeclares a list of Units to be imported
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Example code : Creating a car class from a vehicle interface
// Full Unit code.
// -----------------------------------------------------------
// You must store this code in a unit called Unit1 with a form
// called Form1 that has an OnCreate event called FormCreate.

unit Unit1;


  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, DateUtils, StdCtrls;

  // An interface definition
   IRecyclable = Interface(IInterface)
    // A single function supporting the property
     function GetIsRecyclable : Boolean;
    // A single property
     property isRecyclable : Boolean read GetIsRecyclable;

  // Define our Car class
  TCar = class// (TInterfacedObject, IRecyclable)
    carName  : String;
    carAge   : Byte;
    carIsRecyclable : Boolean;
    function GetIsRecyclable : Boolean;  // Added for IRecyclable
    // Car properties
    property Name : String read carName;
    property Age : Byte read carAge write carAge;

    // Added for IRecyclable
    property isRecyclable : Boolean read GetIsRecyclable;

    // Car constructor
    constructor Create(name : String);

  // Define our Bicycle class
  TBicycle = class// (TInterfacedObject, IRecyclable)
    bikeIsMale       : Boolean;
    bikeWheelSize    : Byte;
    function GetIsRecyclable : Boolean;  // Added for IRecyclable
    // Bicycles properties
    property isMale : Boolean read bikeIsMale;
    property wheelSize : Byte read bikeWheelSize write bikeWheelSize;

    // Added for IRecyclable
    property isRecyclable : Boolean read GetIsRecyclable;

    // Bicycle constructor
    constructor Create(isMale : Boolean; wheelSize : Byte);

  // The definition of the program form
  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
    { private declarations }
    { public declarations }

  Form1: TForm1;


{$R *.dfm}

// Constructor implmentation for the car class
constructor TCar.Create(name : String);
  // Save the car name and set default age and miles
  carName         := name;
  carAge          := 0;
  carIsRecyclable :=true    // Sets the recyclable indicator

// Car function required for IsRecyclable attribute
function TCar.GetIsRecyclable : Boolean;
  Result := carIsRecyclable;

// Constructor implmentation for the bicycle class
constructor TBicycle.Create(isMale : Boolean; wheelSize : Byte);
  // Save the passed parameters
  bikeIsMale    := isMale;
  bikeWheelSize := wheelSize;

// Bicycle function required for IsRecyclable attribute
function TBicycle.GetIsRecyclable : Boolean;
  // We'll asy that only male bicycles are recyclable
  if self.isMale
  then Result := true
  else Result := false;

procedure TForm1.FormCreate(Sender: TObject);
  mumsBike : TBicycle;
  dadsCar  : TCar;
  // Instantiate our bike and car objects
  mumsBike := TBicycle.Create(false, 24);
  dadsCar  := TCar.Create('Nissan bluebird');

  // Ask if each is recyclable
  if dadsCar.isRecyclable
  then ShowMessage('Dads car is recyclable')
  else ShowMessage('Dads car is not recyclable');

  if mumsBike.isRecyclable
  then ShowMessage('Mums bike is recyclable')
  else ShowMessage('Mums bike is not recyclable');

  Dads car is recyclable
  Mums bike is not recyclable
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