Meaning of Apostasy

Apostasy refers to the state of having rejected the beliefs of a religion or favoring the beliefs of other religion instead of yours. Apostasy in Islam is normally known as ‘Riddah’ or ‘Irtidad’ and is defined as the rejection of a word or deed of his religion (Islam) and has changed his beliefs. The punishment of apostasy in Islam is death or imprisonment until repentance for a person who is adult and have a sound mind.
 

References From Quran on Apostasy

•   They but wish that ye should reject Faith, as they do, and thus be on the same footing (as they): But take not friends from their ranks until they flee in the way of Allah (From what is forbidden). But if they turn renegades, seize them and slay them wherever ye find them. (Qur’an 4:89)

•   Why should ye be divided into two parties about the Hypocrites? Allah hath upset them for their (evil) deeds. Would ye guide those whom Allah hath thrown out of the Way? For those whom Allah hath thrown out of the Way, never shalt thou find the Way. (Quran 4: 88)

•   Except those who join a group between whom and you there is a treaty (of peace), or those who approach you with hearts restraining them from fighting you as well as fighting their own people. If Allah had pleased, He could have given them power over you, and they would have fought you: Therefore if they withdraw from you but fight you not, and (instead) send you (Guarantees of) peace, then Allah Hath opened no way for you (to war against them).  (Qur'an 4:89)

References From Hadith on Apostasy

•   It was narrated that Tameem al-Daari said: “I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: ‘This matter (Islam) will certainly reach everywhere that night and day reach, and Allah will not leave any house or tent (all dwellings, in towns and in the desert), but Allah will cause this religion to enter it, and some people will be honored because of it [by accepting it] and others will be humiliated because of it [for rejecting it], and they will be ruled by the Muslims, an honor which Allah will bestow on Islam and a humiliation which He will inflict on kufr (disbelief).

(Narrated by Ahmad, 16344; classed as saheeh by al-Albaani in al-Silsilah al-Saheehah, 3).
 

•   Narrated 'Abdullah: The Prophet said, "The blood of a Muslim, who confesses that Lâ ilâha ill-Allâh (there is no god but Allah), cannot be shed except in three cases:

1. Life for life (in cases of intentional murders without right i.e., Law of Equality in punishment).
2. A married person who commits illegal sexual intercourse.
3. The one who turns renegade from Islam (apostate) and leaves the group of Muslims. 

Sahih al-Bukhari, 9:83:17

•   Abdullah (b. Mas'ūd) reported Allah's Messenger as saying: It is not permissible to take the life of a Muslim who bears testimony (to the fact) that there is no god but Allah, and I am the Messenger of Allah, but in one of the three cases: the married adulterer, a life for a life, and the deserter of his Din (Islam), abandoning the community. Sahih Muslim, 16:4152
   

•   [In the words of] Allah's Apostle, 'Whoever changed his Islamic religion, then kill him. Bukhari, 84:57

• A man who embraces Islam, then reverts to Judaism is to be killed according to "the verdict of Allah and his apostle. Bukhari 89:271

• There was a fettered man beside Abu Muisa. Mu'adh asked, 'Who is this (man)?' Abu Muisa said, 'He was a Jew and became a Muslim and then reverted back to Judaism.' Then Abu Muisa requested Mu'adh to sit down but Mu'adh said, 'I will not sit down till he has been killed. This is the judgment of Allah and His Apostle (for such cases) and repeated it thrice.' Then Abu Musa ordered that the man be killed, and he was killed. Abu Musa added, 'Then we discussed the night prayers. Bukhari 84:58

What Constitute Apostasy in Islam

The conditions which are considered while analyzing the person in question are

1. Person has understanding of shahada

2. He have knowledge of shariah rulings which should be known by all muslims

3. Person should be of sound mind

4. Person should be an adult.

5. Person has consciously & deliberately rejected the beliefs of Islam which includes the rulings of shariah commonly known by all Muslims.

Punishment for Apostasy in Islam

In Islam the male who commit Apostasy i.e. Apostate must be punished to death unless he is suffering from mental disorder or committed this under duress. On the other hand Female Apostate must be put to death or either imprisoned unless she reverts back to Islam.