What does being a "parent" mean?

CPA04

Darth's post got me thinking, what does being a "parent" mean? 

XH sees B twice a month and likes to "do stuff" with him.  It reminds me of a child being in the Big Brothers/Big Sisters program where someone takes the child to do a fun thing a couple times a month.  He doesn't "parent" him.  He doesn't provide for him financially, doesn't discipline him, he'll never be present at school conferences, won't be taking him to flag football practice or swim lessons.  Although he's B's father and I know he loves him, sometimes it's hard to say he's B's parent.

What does it mean to be a parent?