Linguistically means to merge or to insert.
In Tajweed/Quran, Idghaam occurs when two letters meet, and one of them is inserted into another one, and then what remains is the second letter because it has stronger characteristics then the first letter.
Doing Idghaam or Merging, think highways, before you merge to a highway there are two lanes and once you merge it becomes one.
SO the little lane joins/merges into bigger, main highway.
Just like in highways, city roads become part of the bigger highways, the first letter merges into the second letter, and in the process loses more of its original characteristics.
So the two letters in quran, first one mergin into 2nd one completely or incompletely.
So, if noon sakina comes at the end of the word and one of the following letters (يرملون) comes in the beginning of the 2nd word, then there is idhghaam.
These letters are: يرملون yarmaloon or ينمو , ر , ل
Two types of Idghaam- إدغام:
Idhgham bi-ghunnah (i.e. idhgham with nasalization), can be further divided in two categories: a) idhgam al-naqis and b) idhgham al-kamil.
a) الاٍدغام ناقص بغنٌة
Incomplete merging, something remains from the noon, so what remains back?
The nasalization remains back from the noon, hence the name Idgham bighunnah or idgham naqis.
It occurs in letters: و and ي
Becase of a shaddah, it becomes 2 harakaat long.
Examples: من يٌعمل mayy-yamal (add shaddah to the yaa’, shaddah and idgham together makes it 2 sec long)
من ولي, ولي ولا– waliyeww-wala, meww-waliyun
b) الاٍدغام الكامل بغنٌة
Here al-kamil means completed merging with nasalization, because meem and noon are already nasalized (it’s a property in them) and they are completely pronounced thus, klamil.
This occurs in letters ن and م
Examples: مٌال من , كل من– kullumm-min, mimm-maa
من نٌعمة – minn-ni3matin.
Most of the letter noon is not said, so you don’t give this letter full right, it should touch the upper jaw roof.
2) إدغام بغير غنٌة
Idhgham bi-ghairi ghunnah (i.e. idhgham with nasalization) has only one kind, Idhgham al-kamil.
· الاٍدغام الكامل
الكامل– Kamil means complete, i.e. complete merging. When noon as-sakina or tanween is followed by a laam or raa, then there is idghaam kamil, that is, the first letter is gone completely, and is not even there anymore.
Logic: Noon has natural nasalization/ghunnah in it, but in this case one disappears completely, also called bi-ghairi ghunnah, without nasalization (combining without nasal sound). It occurs in letters : ل and ر
من ربهم broken down as ربهم منْ, pronounced as مرٌبهم – mirrabihim.
What remains is the shaddah. As shaddah is idgham!
توابا رحيما – tanween and the raa’
من لٌدنه – milladunhu, all remains is the shaddah.
سائغا للشاربين – the laam is not original, saa’eeghallissharebeen.
1) It has to be within two words. (idhaar can be within one word or two words). Four words in quran that has the letters in it : بنيانا , الدنيا, قنوان , صنوان . So, we change the meaning if we take away the noon, so these are exceptions in the same word. And you won’t find examples other than these, that have letters from yarmaloon with noon sakina/tanween.
2) Within two words: ن, ن والقلم , the 7ukm (Hukm) is to say the harf noon clearly, e.g. يس والقرآن, also in saktah latifah, من راق, man raaq.
Perhaps, the following website will clear the issue much better:
Sources: The above notes are taken from an online halaqah on tajweed (which started recently), and the teacher is using the book “بعض المفيد في علم التجويد“, and few websites.
(Note: I have edited this post, and it is quite different from the original one. This way of categorizing is much easier to remember and understand. Therefore, for the sake of simplicity, I have edited the post, and the contents of course comes from the tajweed classes hosted at IIT )