Jazakallah

Jazāk Allāh (Arabic: جَزَاكَ ٱللَّٰهُ, jazāka -llāh) or Jazāk Allāhu Khayran (جَزَاكَ ٱللَّٰهُ خَيْرًا, jazāka -llāhu khayran) is a term used as an Arabic expression of gratitude, meaning "May God reward you [with] goodness."

Although the common word for thanks is shukran (شُكْرًا), Jazāk Allāh khayran is often used by Muslims instead.

The response to this phrase is wa ʾiyyāk(i) (وَإِيَّاكَ), or wa ʾiyyākum (وَإِيَّاكُمْ) for the plural, which means "and to you". A more formal reply is "wa ʾantum fajazākumu llāhu khayran" (وَأَنْتُمْ فَجَزَاكُمُ ٱللَّٰهُ خَيْرًا) "And you too, may God reward you with goodness".

• Key to Islamic Terms Archived 2013-02-23 at the Wayback Machine
• Jazak Allahu Khairan Meaning and Answer