keep getting an error where it says "Unit" was not declared

[Airyk]

Everytime i compile my class it keeps telling me that "Unit" and "user" were not declared in this scope and i cannot figure out what the problem is.
here is my header file for the class

  #ifndef _Item_included_
#define _Item_included_

#include <string>
#include "Entity.h"

using namespace std;

class Item: public Entity
{
public:
        virtual void setWeight(int  nWeight);
        int  getWeight();
        virtual void setValue(int nValue);
        int getValue();
        virtual void setQuantiy(int nQuantity);
        int getQuantity();
        virtual void use(Unit &user);

private:
        int m_nWeight;
        int m_nValue;
        int m_nQuantity;
};

#endif

and the .cpp file

#include "Item.h"
#include <string>

using namespace std;

int Item::getWeight()
{
        return m_nWeight;
}

void Item::setWeight(int nWeight)
{
        m_nWeight = nWeight;
}

int Item::getValue()
{
        return m_nValue;
}

void Item::setValue(int nValue)
{
        m_nValue = nValue;
}

int Item::getQuantity()
{
        return m_nQuantity;
}

void Item::setQuantity(int nQuantity)
{
        m_nQuantity = nQuantity;
}

void Item:: use(Unit &user)
{
        nQuantity--;
        nValue--;
        nWeight--;
}

[keskiverto]

What and where is the definition of Unit?