This is most likely the DC-IN port. According to which Book you have, here is the most likely part: http://www.ifixit.com/iBook-Parts/iBook-G4-12-Inch-DC-In-Board/IF183-030
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This is most likely the DC-IN port. According to which Book you have, here is the most likely part: [product|IF183-030]
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Just click on the "Guides" link to the right to find the right part and installation instructions.
This is most likely the DC-IN port. According to which Book you have, here is the most likely part: http://www.ifixit.com/iBook-Parts/iBook-G4-12-Inch-DC-In-Board/IF183-030