What is the meaning of saying "amin" after dua?

Question

Why do we say Amin after doing a dua (praying) ? What does Amin mean?
Christians say a similar word; namely Amen. Is this only a coincidence?

Answer 1

Meaning & Derivation of Aameen ( āmīn / آمين‎ )

Scholars are divided over the source of the word Aameen.

• Some say it is the abbreviation of the word Allaahummastajib lana which means
  Oh Allah accept our invocation
  or
  O Allah, respond to, (or answer) what we have said.

• Other scholars say it is derived from the word Āmana (Arabic: آمن‎) which means So be it. (Even Wikipedia agrees to it)

Importance of Aameen

TL;DR (Too long ; Didn't read)(Short version)

There are many hadees which state the importance of Aameen. Some say that if our saying Aameen coincides with that of the angels our previous sins get washed away (you need serious timing here :P)
Also a few hadiths say that Prophet Muhammad (PBUH) used to say it after finishing Surah Fatihah and he used to say it loudly.

Long Version

Saying it is a sunnah. It means you can chose not to do it.

Hadees about Aameen -

Bukhari :: Book 6 :: Volume 60 :: Hadith 2

Narrated Abu Huraira:

Allah’s Apostle said, “When the Imam says: ‘Ghair-il-Maghdubi ‘Alaihim Walad-Dallin (i.e. not the path of those who earn Your Anger, nor the path of those who went astray (1:7)), then you must say, ‘Ameen’, for if ones utterance of ‘Ameen’ coincides with that of the angels, then his past sins will be forgiven.”

Another Hadith

Muslim :: Book 4 : Hadith 828

Abu Huraira reported: The Messenger of Allah (may peace be upon him) while teaching us (the principles of faith), said: Do not try to go ahead of the Imam, recite takbir when he recites it. and when he says:" Nor of those who err," you should say Amin, bow down when lie bows down, and when he says:" Allah listens to him who praises Him," say:" O Allah, our Lord, to Thee be the praise".

For those who dont know Nor of those who err is the last line of Surah Fatihah.

Another one

Bukhari :: Book 1 :: Volume 12 :: Hadith 747

Narrated Abu Huraira:

The Prophet said, "Say Amin" when the Imam says it and if the Amin of any one of you coincides with that of the angels then all his past sins will be forgiven." Ibn Shihab said, "Allah's Apostle used to Say "Amin."

Hadees no 748 & 749 are the same as this one.
And hadees

Malik :: Book 3 : Hadith 3.12.47

Yahya related to me from Malik from Ibn Shihab that Sa'id ibn al-Musayyab and Abu Salama ibn Abd ar-Rahman told him from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, "When the imam says 'Amin', say 'Amin', for the one whose 'Amin' coincides with the 'Amin' of the angels - his previous wrong actions are forgiven him."

Ibn Shihab said, "The Messenger of Allah, may Allah bless him and grant him peace, used to say 'Aameen' (extending it)."

And another hadees

Dawud :: Book 3 : Hadith 932

Narrated Wa'il ibn Hujr:

When the Apostle of Allah (peace_be_upon_him) recited the verse "Nor of those who go astray" (Surah al-Fatihah, verse 7), he would say Amin; and raised his voice (while uttering this word).

Hadees no 934 is th same just the following line is added

he would say Amin so loudly that those near him in the first row would hear it.

Another hadees

Dawud :: Book 3 : Hadith 938

Narrated AbuZuhayr an-Numayri:

AbuMisbah al-Muqra'i said: We used to sit in the company of AbuZuhayr an-Numayri. He was a companion of the Prophet (peace_be_upon_him), and he used to narrate good traditions. Once a man from among us made a supplication. He said: End it with the utterance of Amin, for Amin is like a seal on the book.

AbuZuhayr said: I shall tell you about that. We went out with the Apostle of Allah (peace_be_upon_him) one night and came upon a man who made supplication with persistence. The Prophet (peace_be_upon_him) waited to hear him. The Prophet (peace_be_upon_him) said: He will have done something which guarantees (Paradise for him) if he puts a seal to it. One of the people asked: What should he use as a seal? He replied: Amin, for if he ends it with Amin, he will do something which guarantees (Paradise for him).

Then the man who questioned the Prophet (peace_be_upon_him) came to the man who was supplicating, and said to him: So-and-so, end it with Amin and receive the good news. These are the words of Mahmud.

Another hadees (im just posting a part of it)

Dawud :: Book 3 : Hadith 967

Narrated AbuMusa al-Ash'ari:

He said: When you pray a (congregational) prayer, straighten your rows, then one of you should lead you in prayer. When he says the takbir (Allah is Most Great), say the takbir, and when he recites verses "Not of those upon whom is Thy anger, nor of those who err" (i.e. the end of Surah i.), say Amin; Allah will favour you. When he says "Allah is most great," and bows, say "Allah is most great" and bow, for the imam will bow before you, and will raise (his head) before you.

There are a few more but that would make the answer too long.
Most of them say the same thing that it is important to recite it and if your Aameen coincides with that of angels your past sins will be forgiven.

Coincidence of the word Aameen in other religions.

This is not some coincidence. Basically for them it used to mean the same thing when the Gospel was given to Jesus (PBUH). They used to say it after they said their prayers (dua). (i dont know what it means now days)

Also the Jews say that. Moses (PBUH) also used to use it after his prayers (dua).

Answer 2

According to majority of the scholars, Aameen is an abbreviation of 'Allaahummastajib lana' (Oh Allah! Accept our Duaa (supplication)).

Not only the christians, the Jews too use it at the end of their prayers. The biggest reason of this common word is very obvious: All are ahl al kitab (I have done no research on this and am telling this from my side). Another reason is Arabic itself. If you look at http://www.wikipedia.org/wiki/Amen you'll find the following lines:

the basic triconsonantal root from which the word was derived is common to a number of Semitic Languages such as Aramaic and Arabic.

Also read the page http://maseeh1.tripod.com/advices7/id216.htm for more. Please confirm the hadith from the page with the scriptures.

And verily Allah knows best. . . .

Jazaakallah