What is "Partial birth abortion"?

Factual Questions
1

I did a search and found much debate but was unable to find a definition.
Could someone please give me a objective, clinical explanation?
Several friends and I have wondered since the State of the Union speech.

2

Here’s a recent thread on this subject with a lot of good information:

http://boards.straightdope.com/sdmb/showthread.php?threadid=156829