A second cousin once removed is either a parent's second cousin or a second cousin's child. Both of these people fit the description of this relationship.
The word "removed" refers to the fact that the person and the second cousin are one generation apart, while true second cousins are in the same generation. While it may seem that a second cousin once removed should be called a third cousin, third cousins are actually the children of second cousins once removed. You and your third cousin share a great-great-grandparent, while your great-great-grandparent is the great-grandparent of your second cousin once removed.