The south-western corner of the Holy Ka‘bah. During tawaf, this corner should be touched when possible. Kissing it is prohibited. It is narrated in a hadith that Ibn ‘Umar (may Allah be pleased with him) said, “I never saw the Messenger of Allah (ﷺ) touch any part other than the two Yamani corners.”[1]
[1] - لم أر رسول الله صلى الله عليه وسلم يمسح من البيت إلا الركنين اليمانيين (Muslim: 2/924)