particle physics - Why absence of electron is called hole?

I am having hard time in understanding the concept of holes:

1. 
   

   If there is no electron than how can it be a hole?

   
   



2. 
   

   For a moment lets assume absence of electron is termed as hole but how can this absent particle have mass?

   
   


3. 
   

   Under equilibrium no electrons are present in conduction band. Why cant we termed this absence as holes in conduction band?