Al-Aqsaa Mosque also known as Al-Aqsaa and Bayt ul-Muqaddas, is the third holiest site in Islam and is located in the Old City of Jerusalem. The site on which this silver-domed mosque sits, along with the “Dome of the Rock”, is referred to as al-Haram ash-Sharif ("the Noble Sanctuary"). 

The Organisation of Islamic Cooperation, refers to the al-Aqsaa Mosque as the third holiest site in Islam (and calls for Arab sovereignty over it).

The Glorious Qur’an clarifies that Prophet Muhammad (ph&hp) was transported from the Sacred Mosque in Makkah to al-Aqsaa Mosque (in Al-Quds) during the well-known miraculous “Night Journey”:

Absolute Pure is the One Who took His devoted worshipper [Prophet Muhammad] for a Divine Excursion from Masjid-al-Haram to Masjid-al-Aqsaa, the neighborhood of which We have blessed, in order to show him Our Signs of Power.  Indeed Allah is the Seeing Hearer.

Surah Isra (17), Ayah 1

In Islam, the term "al-Aqsaa Mosque" refers to the entire Noble Sanctuary. This mosque is the second house of prayer constructed after the Masjid al-Haraam in Makkah.

The historical significance of the al-Aqsa Mosque in Islam is further emphasized by the fact that it was the first Qiblah of Muslims.  They faced al-Aqsaa Mosque during their prayers until Prophet Muhammad (ph&hp) received revelation by Allah to turn towards Ka’ba (in Makkah) instead of Masjid ul-Aqsaa in Al-Quds (Ayah 144, Surah Al-Baqarah (2)).