Heb. rahum”The Merciful.” One of the ninety-nine names or attributes of God. It generally occurs in conjunction with the ar-Rahim, e.g., Surah ii. 159: “Your God is one God. There is no god but He, the Merciful, the Compassionate” It also occurs in the initial formula. placed at the commencement of each Surah, with the exception of the ixth. “In the name of God, the Merciful, the Compassionate”
Al-Baizawi says that ar-Rahman is a more exalted attribute than ar-Rahim, because it not only contains five letters whilst Rahim only has four, but it expresses that universal attribute of mercy which the Almighty extends to all mankind, the wicked and the good, believers and unbelievers.
Based on Hughes, Dictionary of Islam