const = 0 после прототипа функции что означает?

в библиотеке qwt, в файле qwt_series_data.h, объявлен класс: QwtSeriesData. Что означает virtual size_t size() const = 0;, а именно = 0?

template <typename T>
class QwtSeriesData
{
public:
    //! Constructor
    QwtSeriesData();
    //! Destructor
    virtual ~QwtSeriesData();
    //! \return Number of samples
    virtual size_t size() const = 0;
    /*!
      Return a sample
      \param i Index
      \return Sample at position i
     */
    virtual T sample( size_t i ) const = 0;
    /*!
       Calculate the bounding rect of all samples
       The bounding rect is necessary for autoscaling and can be used
       for a couple of painting optimizations.
       qwtBoundingRect(...) offers slow implementations iterating
       over the samples. For large sets it is recommended to implement
       something faster f.e. by caching the bounding rectangle.
       \return Bounding rectangle
     */
    virtual QRectF boundingRect() const = 0;
    /*!
       Set a the "rect of interest"
       QwtPlotSeriesItem defines the current area of the plot canvas
       as "rectangle of interest" ( QwtPlotSeriesItem::updateScaleDiv() ).
       It can be used to implement different levels of details.
       The default implementation does nothing.
       \param rect Rectangle of interest
    */
    virtual void setRectOfInterest( const QRectF &rect );
protected:
    //! Can be used to cache a calculated bounding rectangle
    mutable QRectF d_boundingRect;
private:
    QwtSeriesData<T> &operator=( const QwtSeriesData<T> & );
};