Points of a plot brought up early in the story in seemingly an important way, and totally forgotten or unaddressed by the end. The "Exact Duplicate of Earth" comment in the TOS episode "Miri" is a perfect example. The teaser specifically mentions the world is a duplicate of Earth, and promptly drops the issue. Plot orphans are sloppy writing and/or sloppy editing at work. (Garry Stahl)
The booster plot should not be mistaken for the plot orphan.