What is holiness?

Holiness means to be set apart. God says in the Bible that he is a holy God. Therefore, he is like no other because there are no other real gods, just gods that people create.
Since God is holy then anything he touches is also holy. Remember when God met Moses with the burning bush, God told him to take off his sandals because where he was standing was holy.
This idea also transfers to us. If God has touched our lives through Jesus Christ then we are holy also. God said so in the Bible, "Be holy as I am holy." That means we should live a lifestyle that is irreproachable, one that shows we have God in our lives, not just mere religion. God expects a lifestyle from us if we are sincere.
== My own interpretation ==
Absence of greed, jealousy, covetousness, avarice, pride, ego, anger, lying, slander, hatred, attachment, shamelessness, etc.

Answer

The Holy Spirit is the one who is the source of power and guidance for those who wish to exhibit true holiness (as mentioned above). Without his indwelling and abiding presence we are essentially left to do it ourselves. We then tend to reduce holiness down to areas we feel capable of achieving, which then leads to pride in self achievement which is sin and not holiness.



NO man can determine what acts, thoughts and lifestyle are Holy. It is only with the aid of the Holy Spirit we can do this, for the Lord says 'your ways are not my ways'. Our conscious level of good is not the Lord's standard. He is the ONLY one that can transform, teach and remind us of how we are meant to live.

 

A Scriptural perspective:

The New Testament uses the word "holiness" in different ways, and it doesn't always mean exactly the same thing. In fact, different Greek word forms were used in the original texts, though they're all translated to "holiness" in English. The following verses provide examples of the various shades of meaning:

Luke 1:75 - In holiness and righteousness before Him all the days of our life.

Here, "holiness" (Greek hosiotes) means "piety towards God; fidelity in observing the obligations of piety; demonstrating proper reverence and obedience."

Romans 6:19 - I speak in human terms because of the weakness of your flesh. For just as you presented your members as slaves of uncleanness, and of lawlessness leading to more lawlessness, so now present your members as slaves of righteousness for holiness.

In this verse, the Greek word is hagiasmos, meaning "purification; consecration." It's in this sense that it includes being "set apart."

2 Corinthians 7:1 - Therefore, having these promises, beloved, let us cleanse ourselves from all filthiness of the flesh and spirit, perfecting holiness in the fear of God.

The Greek word this time is hagiosune, meaning "moral purity."

Hebrews 12:10 - For they indeed for a few days chastened us as seemed best to them, but He for our profit, that we may be partakers of His holiness.

This verse, speaking of the holiness of God Himself, uses the Greek form hagiotes, meaning "sanctity;" the idea being that God is completely "set apart," but that He gives man the chance to become a "partaker," or "have a share in," His holiness.

[Quotes from NKJV]