What is maddu albadal

Question
What is maddu albadal
Answer
The brief answer is that medd al badl is a hamzah followed by a medd letter in the same word.  The detailed explanation is found in the lesson in the link below:
http://www.abouttajweed.com/tajweed-rules/49-the-mudood-the-lengthenings/106-the-mudood-lengthenings-part-4