From Google Search, a few references to and definitions of Holiness:

Spiritually perfect or pure; untainted by evil or sin; sinless; saintly.

Regarded with or deserving deep respect, awe, reverence, or adoration.

Living or undertaken with highly moral or spiritual purpose; saintly.

A holy person; a holy way of life.

Belonging to, derived from, or associated with divine power; sacred.

Separated or set apart from (something unto something or someone else).

Regarded as deserving special respect or reverence.

The pursuit of peace is our holiest quest.

Dedicated to religious use; belonging to or coming from God; consecrated; sacred.

A holy thing or place.

The definition of holy is religiously sacred or dedicated to God and religion.

An example of holy used as an adjective is the phrase Holy Eucharist which means the bread and body of Christ given at Communion.

An example of a holy person is Mother Theresa.

Set apart or dedicated for a specific purpose, or for use by a single entity or person.

And this one:

The Bible says, “Give unto the LORD the glory due unto his name; worship the Lord in the beauty of holiness” (Ps. 29: 2, 1 Chron. 16: 29; Ps. 96: 9).

What does it mean to worship God and His Son, in the beauty of holiness? I think that first, it is recognizing that God is holy, that he is totally pure, totally perfect in all things, and is set apart from all things wrong. We also have to realize that He wants us to be like him in that regard. What? That is impossible. To be like Him? He wants us to be Holy? How do we become that? I can tell you what doesn’t make us holy. Religion with its rituals, chanting words, and repeating mantras, those things mean nothing to God. God looks into the heart. He looks for a pure heart, a child-like heart, for true repentance and confession not just for words out of our mouths. We have all sinned and before we can worship in Holiness we must “get right” with Him. That is not impossible to do. We simply confess that we are sinners and want to not be one any longer. We want our prayers to him to come straight out of our souls, to not pray from the shadows, but from the realization that we are covered by the Blood of the Lamb, and through Jesus Christ, we can worship God in holiness. It is amazing when you realize that Real Prayer is not a waste of time, but opens up a whole new experience for us. And what is that? Having prayers answered. They are answered because we pray with confidence, knowing that God hears us, just as he heard Samuel and David and Moses. Let me say that again, we can call on His name knowing that He hears us and wants to answer us.

So when trouble comes to our lives and believe you me, we all have them, hope should rise up within you that God will hear the prayers from a repentant heart, that we mean business when it comes to believing in Him. As a pastor once said, “God does business with those who mean business, so pray, and get ready to receive.”

Psalm 99

1 The Lord reigns,
    let the nations tremble;
he sits enthroned between the cherubim,
    let the earth shake.
2 Great is the Lord in Zion;
    he is exalted over all the nations.
3 Let them praise your great and awesome name—
    he is holy.

4 The King is mighty, he loves justice—
    you have established equity;
in Jacob you have done
    what is just and right.
5 Exalt the Lord our God
    and worship at his footstool;
    he is holy.

6 Moses and Aaron were among his priests,
    Samuel was among those who called on his name;
they called on the Lord
    and he answered them.
7 He spoke to them from the pillar of cloud;
    they kept his statutes and the decrees he gave them.

8 Lord our God,
    you answered them;
you were to Israel a forgiving God,
    though you punished their misdeeds.[a]
9 Exalt the Lord our God
    and worship at his holy mountain,
    for the Lord our God is holy.