What is the definition of Ghunnah and Ikhfah? What is the difference between them

What is the definition of Ghunnah and Ikhfah. What is the difference between them with examples. In Tajweed books sometime it is written to do Ikhfah and sometime they tell to do Ghunnah, is it one of the same or there is a difference in that.

The ghunnah is a nasal sound emitted from the nose.  The ikhfa' linguistically means: hiding or concealment.  In applied tajweed the ikhfa' is defined as: The pronunciation of a non-voweled letter stripped of any shaddah, characterized somewhere between an and an  with a ghunnah remaining on the first letter.
As you can see by the definition of the ikhfa', the ghunnah accompanies it, but is not part of the letter being pronounced between an and .   In fact, the ghunnah sound (nasalization) accompanies every and regardless whether the rule is ikhfa' or otherwise and regardless whether the and are saakinah or voweled. 
So when you have the ikhfa' rule -which only occurs with the and saakinah when followed by specific letters- we also have a ghunnah accompanying it, meaning a nasalization from the nose.  The ghunnah though can be heard with other rules of the  and saakinah.  The ghunnah is an inherent characteristic of both the and .
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Wa iyyakum