خير
Khayr
The good, meaning whatever is good in the sight of God, including wealth, deeds, and what leads a person to Him.
What it means
Khayr is the general Arabic word for good and for what is better. The Quran uses it for moral good: “Whoever does an atom’s weight of good will see it” (99:7). It also uses it for the best course of action, as in the call to “invite to the good” (3:104), and for the guidance that is sent from God (2:105). In a few places it is used for wealth, as in the verse that speaks of leaving khayr for one’s heirs (2:180).
Muslims use the word in everyday speech too. A person may say jazāk Allāhu khayran, “may God reward you with good”, as thanks, and inshāʾ Allāh khayr, “if God wills, it is for the good”, when the outcome is not yet known. In both cases the speaker treats God as the source of whatever good comes, which matches the Quranic use of the word.
Often misunderstood as
A single English word such as “good” hides that khayr is measured by what God has commanded and approved, and not by custom or personal taste.
What the sources say
The Quran says that a person may dislike a thing in which God has placed much khayr (4:19), and it teaches that good which is done for God is never lost (3:115).
Term and Arabic as listed in Ismail al Faruqi, Toward Islamic English (International Institute of Islamic Thought, 1995), p. 31, listed as KHAYR. The definition is restated here.