al-Quddūs

The One who is pure, spotless, without blemish or fault.

The One who is far from, and untouched by, worldly imperfections or faults.

The One whose essence and attributes are of unimaginable purity and perfection.

The One who is beyond all human understanding of purity, perfection and holiness.

From the root q-d-s which has the following classical Arabic connotations:

to be pure, clean, spotless
to be far removed from impurity or imperfection
to be holy, sacred, hallowed

This name is used in the Qur'ān. For example, see 59:23

The ancient roots of the Arabic word Quddūs, as well as the Hebrew word Kadosh, point toward that which is set apart, different, that which is 'other'. Thus, al-Quddūs is known as the One who is set apart, distinctly different and separate from all worldly imperfections, sins and faults, the One whose perfection and righteousness are so 'other' that they cannot be grasped by mankind, the truly Blessed One.

The name al-Quddūs describes the unique, unimaginable purity and perfection that is Allāh, the One whose goodness and righteousness are beyond our understanding.

According to al-Qurturbi, the name al-Quddūs also describes the One who is glorified and revered by the angels.

The Merriam-Webster dictionary defines the word holy as:

Holy -- Exalted or worthy of complete devotion as one perfect in goodness and righteousness.