Allahumma

The invocation form of the name Allah, used to call on God in worship and prayer.

What it means

Allāhumma is the form of the name Allāh used in direct address, and grammarians explain the final -umma as standing for the call yā. It is used to open a supplication. The Quran has it in the prayer, “O God, Master of sovereignty” (3:26), and in the prayer of Jesus, “O God, our Lord, send down to us a table” (5:114). Muslims say it at the start of a duʿāʾ, and it occurs in the opening supplication of the ritual prayer, subḥānaka llāhumma wa bi-ḥamdika.

Term and Arabic as listed in Ismail al Faruqi, Toward Islamic English (International Institute of Islamic Thought, 1995), p. 23, listed as ALLAHUMMA. The definition is restated here.