Mission to Mars Definition

pixel (pik-sel):  
   Function: noun
   Etymology: pix + element

   1 : any of the small discrete elements that together constitute an
   image (as on a television screen)

   2 : any of the detecting elements of a charge-coupled device used as
   an optical sensor

Images on computer screens and televisions are made of pixels.  
They are the spots of brightness or color that combine to form an image.