What constitutes intercourse?
Question

                Ghusl is required if intercourse takes place, but what is meant by intercourse? Is any kind of foreplay regarded as intercourse?.
Praise be to Allah.
    Not all kinds of foreplay are regarded as intercourse; rather 
    intercourse is when the tip of the penis disappears fully into the vagina. 
    If that happens, then intercourse has taken place; if no penetration occurs 
    or if only part of the tip penetrates but not all of it, then this is not 
    intercourse. This is what is indicated by the ahaadeeth: 

    It was narrated from Abu Hurayrah (may Allaah be pleased with 
    him) that the Prophet  (peace and blessings of Allaah be upon him) said: 
    “When a man sits between the four parts (arms and legs of his wife) and has 
    intercourse with her, then ghusl is obligatory.” Narrated by al-Bukhaari, 
    291; Muslim, 525. 

    The phrase translated as “[has intercourse with her]” is a 
    metaphor for inserting the penis into the vagina. This was stated by 
    al-Haafiz in al-Fath. 
The Messenger of Allaah 
     (peace and blessings of Allaah be upon him) said: “When a man sits 
    between the four parts (arms and legs of his wife) and the one circumcised 
    part touches the other, then ghusl is obligatory.” Narrated by Muslim, 349. 

    Al-Nawawi said in Sharh Muslim: 
With regard to the 
    phrase “and the one circumcised part touches the other, then ghusl is 
    obligatory”: the scholars said: what this means is when your penis 
    disappears into her vagina; it does not mean touching in a real sense. That 
    is because the circumcised part of a woman is above the vagina, and the 
    penis does not touch it during intercourse. The scholars are unanimously 
    agreed that if he places his penis on the circumcised part and does not 
    enter (the vagina), then no ghusl is required, neither for him nor for her. 

    The fact that what is meant is what we have mentioned and 
    that what is meant by touching is the two parts coming in line with one 
    another is indicated by the other report, “when the two circumcised parts 
    meet” i.e., coming in line with one another.  

    It says in al-Majmoo’ (2/150): 
The obligation of ghusl 
    and all other rulings that have to do with intercourse are subject to the 
    condition that the tip of the penis disappears completely into the vagina, 
    and they are not dependent on more than that. None of the rulings have to 
    with part of the tip disappearing only. End quote. 

    Al-Haafiz said in al-Fath: 

    What is meant by touching and meeting is coming in line with 
    one another, which is indicated by a report narrated by al-Tirmidhi. It does 
    not mean touching in a literal sense, because that does not happen when the 
    tip of the penis disappears. End quote. 

    Al-Shawkaani said: The ahaadeeth mention the words coming in 
    line with one another, meeting, touching, but what is meant by meeting is 
    coming in line with one another. 

    Al-Qaadi Abu Bakr said: If the tip of the penis disappears 
    into the vagina, then ‘meeting’ has taken place. End quote. 

    Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said:  

    It is well known that the site of circumcision is above the 
    tip. As that is the case, then it cannot touch the site of circumcision of 
    the woman until the tip has penetrated. Hence it is stipulated that for 
    ghusl following intercourse to be obligatory, the tip must disappear. It is 
    narrated in some versions of the hadeeth of ‘Abd-‘Allaah ibn ‘Amr ibn 
    al-‘Aas: “When the two circumcised parts meet and the tip of the penis 
    disappears, then ghusl becomes obligatory.” End quote. Narrate by Ibn 
    Maajah, 611; classed as saheeh by al-Albaani in Saheeh Ibn Maajah. 

    Majmoo’ Fataawa Ibn ‘Uthaymeen, 
    11/323 
Based on this, what is 
    meant by the one circumcised part touching the other and the meeting of the 
    two circumcised parts is when the site of circumcision of the man comes into 
    line with the site of circumcision of the woman, and that happens when the 
    entire tip of the penis disappears into the vagina. If the tip of the penis 
    disappears into the vagina then intercourse has taken place and ghusl is 
    required. 

    And Allaah knows best.