R.J.
As others have said it's normal for breastfed babies to go up to a week to ten days without pooping. As a matter of fact, though, many children and adults are once a week'ers, although that tends to freak once a day'ers out.
It just breaks my heart when I seem parents shoving enemas, shoving fingers up and wiggling them around, forcing milk of magnesia, and forcing fibers on kids just because their digestive systems are different than their parents. Poor traumatized little tykes. (While some children DO get constipated I've seen more terrified kids in the office who just happened to have a totally normal poo schedule that their parents didn't like).
Constipated means that the poop is THERE but is too solid to pass. It comes with a lot of pain and discomfort. An extremely efficient digestive system, however, doesn't create enough poop to eliminate for several days.