– Meaning those matters which make it obligatory (fard) for a person to perform ghusl
– Meaning the appearance
– From the private parts, whether it be whilst awake or when asleep
The semen of a man is thick and white in colour, the semen of women is thin and yellow.
accompanied by desire,
– Meaning with pleasure, even if experienced during an erotic dream.
Therefore if a person finds that there is a flow of semen to the outside of the body due to being struck then ghusl is not compulsory.
Likewise it is a condition for ghusl to be compulsory that it be accompanied by desire at the time of its separating from its place in the body even if it does not appear outside of the body.
inserting of the head of the private part
– It is that which is above the area that is circumcised, even if it is entered with something covering it through which warmth can be sensed according to the correct opinion.
In the front and rear,
– Of a human
upon both of them,
– Meaning the doer and the one to whom it is done, regardless of whether ejaculation takes place
the one awakening to find
– Meaning one awakening knowing, or seeing on himself
– Whether he recalls an erotic dream or not
or pre seminal fluid,
– If he recalls having an erotic dream, due to the possibility that it maybe semen which has thinned due to the atmosphere/weather. It is a thin white fluid which appears when aroused. If a person has an erotic dream and awakes to find not wetness but later pre seminal fluid exits from them, then they do not have to perform ghusl. If however semen was to exit then they must perform ghusl.
the end of menstruation and post birth bleeding
– Meaning after purity from its filth by the ending of the menstrual cycle. If a woman gives birth and does not see any blood it is compulsory for her to perform ghusl according to Imam Abu Hanifah out of precaution, his two students however differed.
 This same ruling also applies to women, see al-Mabsut by al-Sarkhasi (1/70).
 The place of semen in the body of a male is a males back, and a bone near the chest for a female. See al-Dur al-Mukhtar with al-Hashiyah (1/159).
Therefore if the semen appears without being accompanied by desire, such as if the person was to lift a heavy object, or is struck on the back and semen appeared, then they do not have to bathe. He (Taala) said: ‘And if you are junub then purify’, the junub is someone who fulfils his desire. See Fath al-Qadeer (1/41-42).
 Muslim (1/56) reports from Aishah (ra) ‘the Messenger of Allah (s) said: ‘…If the two circumcised parts encountered each other, ghusl is obligatory’.
 Pre Seminal fluid (madhi) is that which appears at the time of foreplay etc.
 The person awakening from their sleep but does not recall having an erotic dream, and finds on their clothing or bedding or body semen, there is agreement that they must perform ghusl.
As for if a person awakes to find pre seminal fluid (madhi), if they recall having an erotic dream there is agreement that they must perform ghusl.
Likewise if they have a doubt if it is semen or pre seminal fluid along with their recalling having had an erotic dream. This is because of the possibility that the fluid that is found may have been semen but has thinned due to the atmosphere/weather, and therefore this fluid is regarded as semen out of precaution.
If the person awakening is certain that the fluid found is pre seminal fluid (madhi) there is agreement that it is not compulsory to perform ghusl.
If the person awakes and does not know if the fluid found is semen or pre seminal fluid and also do not recall having an erotic dream it is compulsory to perform ghusl according to Abu Hanifah and Muhammad bin al-Hasan, because of the possibility of it exiting accompanied by desire, then the person forgot and the semen was thinned by the atmosphere, in difference to the view of Abu Yusuf. The scholars mentioned that Abu Yusufs position is more in conformity with analogy (qiyas) and Abu Hanifah and Muhammad bin Hasans position is more precautionary. The details for this discussion can be found in Fath al-Qadeer (1/42).
 The corresponding fluid excreted by women is called al-qathi as mentioned by al-Zaylai (1/17) and Ibn Abidin (1/122). Al-Shalabi (1/17) mentioned that it is called al-taqthi.
As for al-Wadi: it is a thick fluid which exits after a person urinates. Ibn Abidin (1/122) mentioned that it is a white thick fluid which appears after urinating.
For both al-madhi (pre seminal fluid) and al-wadi ghusl is not required, rather wudu is perfomed. Sharh al-Kanz by al-Zaylai (1/17).
 The evidence for the necessity of ghusl are His (the Exalted) words: ‘Do not go near them until they purify’ [al-Baqarah:222], purifying here refers to bathing (ghusl). Post birth bleeding is regarded being like menstruation due to consensus (ijma) and analogy to menstruation. Fath Bab al-Inayah (1/79).