‘Awrah in Arabic means a weak point along a border and the like.
In Misbaah al-Muneer it says: Everything that a person covers out of dignity or modesty is ‘awrah.
According to the fuqaha’, everything that it is haraam for a man or woman to uncover is ‘awrah.
Covering the ‘awrah, in the terminology of the fuqaha’, means covering that of which exposure is shameful and causes shame, whether the individual is male or female or hermaphrodite.
See: al-Mawsoo‘ah al-Fiqhiyyah, 24/173
Covering the ‘awrah is one of the conditions of prayer being valid because Allah, may He be exalted, says: (interpretation of the meaning):
“Take your adornment (by wearing your clean clothes) while praying”
Ibn ‘Abbaas (may Allah be pleased with him) said: What is meant by adornment in this verse is wearing clothes whilst praying. This was narrated by al-Tabari in his Tafseer, 12/391
And the Prophet (sa) said: “Allah does not accept the prayer of a woman of childbearing age (lit. a woman who menstruates) except with a head covering.”
Narrated by Abu Dawood, 641; narrated and classed as saheeh by al-Tirmidhi, 377; classed as saheeh by al-Albaani.
It says in al-Mughni (1/336):
Concealing the ‘awrah from view with something that does not show the skin is obligatory and is a condition of prayer being valid. This is the view of al-Shaafa‘i and ashaab al-ra’y. End quote.
Ibn Hajar said:
The majority are of the view that covering the ‘awrah is one of the conditions of prayer.
End quote. Fath al-Baari, 1/466
What is required of the worshipper is to cover his ‘awrah whilst praying, according to the consensus of the Muslims. The ‘awrah of a man is the area between the navel and the knees, according to the majority of scholars.
See: al-Mughni, 3/7; al-Istidhkaar, 2/197; Fataawa Islamiyyah, 1/427
With regard to women, the hair and the entire body are ‘awrah and she has to cover them, apart from the face and hands. If she does that, then her prayer is proper, according to scholarly consensus.
See: al-Iqnaa‘ fi Masaa’il al-Ijmaa‘, by Ibn Qattaan, 1/121-123; al-Sharh al-Mumti‘, 2/160 ff.
If a person starts to pray with his ‘awrah covered, then during the prayer he is not sure whether some of it has become uncovered, then he should ignore that doubt and complete his prayer, because the basic principle is that the ‘awrah is still covered, and developing doubt about what you were certain about should not be given any attention.
Al-Bukhaari (137) and Muslim (361) narrated from ‘Abbaas ibn Tameem that his paternal uncle said: A complaint was made to the Prophet (S) about when one thinks that something has happened whilst he praying. He said: “Do not stop until you hear a sound or notice a smell.”
This hadeeth is one of the basic principles of Islam and an important foundation of fiqh, namely that things are deemed to still be as they originally were unless there is certain proof of the contrary, and passing doubts do not affect that. End quote.
What the worshipper has to do is to prepare for his prayer before he starts to pray. So he should put on clothes that he is certain will cover his ‘awrah and leave clothes that he is afraid will let something of his ‘awrah show whilst he is praying, such as short T-shirts and other kinds of clothing which may rise up and show the lower back, lest some part of his ‘awrah be uncovered when he bows or prostrates.