The Arabic word Riddah means Apostasy. It refers to when a Muslim becomes a disbeliever by saying a clear statement to that effect, or by uttering words which imply that (i.e., which imply kufr or disbelief), or he does something that implies that (i.e., an action which implies kufr or disbelief). Linguistically, returning, as indicated in the words of Allah, the Exalted: "And do not (Taraddu) return back." Quran Surah al-Maidah 5: 21
That is, do not revert to infidelity, while Riddah in judicial terminology is apostasy after Islam. Allah, the Exalted, says: "And anyone of you who reverts (Yartadid) from his Deen and dies as an infidel, it is those whose good deeds shall be vain in this world and in the world to come. These are inmates of the Fire, wherein they shall live forever." Quran Surah al-Baqarah 2: 217
Apostasy may take effect by the commission of one of the numerous invalidators of Islam, that they may be summed up in four types:
1: Verbal apostasy, such as reviling Allah, the Exalted, or His Messenger (salallahu alaihe wa-sallam). His angels or any of His Messengers. Or by claiming knowledge of the unseen world, or Prophet hood or believing the claims of such people. Or appealing for help from other than Allah for things that only Allah can do, or seeking refuge with other than Allah.
2: Practical apostasy, such as prostrating before an idol, a stone, a grave, or making offering to them, or by casting the Book of Allah in filthy places, practicing sorcery, learning, or teaching it, or ruling by other than the laws of Allah, and deeming that action is lawful.
3: Doctrinal apostasy, such as believing that Allah has a partner or believing that fornication, consumption of liquor or usury are lawful. Or believing that eating bread is unlawful, or the Salaat is not obligatory or other things whose legitimacy or lawfulness, prohibition or incumbency are unanimously and decisively agreed upon, or commonly known as such.
4. Doubtfulness apostasy, to doubt the illegality of ascribing partners to Allah, prohibition or fornication or consumption of liquor or the lawfulness of consuming bread, or having doubts respecting the Message of the Prophet (salallahu alaihe wa-sallam), or the Message of other Prophets, or suspecting the veracity of the Prophet (salallahu alaihe wa-sallam), or Islam, or doubting the validity of Islam in this age.