From the Complete Word Study Dictionary (emphasis added):
hágios; fem. hagía, neut. hágion (G39), adj. from hágos (n.f.), any matter of religious awe, expiation, sacrifice. Holy, set apart, sanctified, consecrated, saint. It has a common root, hág-, with hagnós (G53), chaste, pure. Its fundamental idea is separation, consecration, devotion to the service of Deity, sharing in God’s purity and abstaining from earth’s defilement.
(I) Pure, clean, ceremonially or morally clean, including the idea of deserved respect, reverence.
(A) It particularly means perfect, without blemish (Romans 12:1).
(B) Metaphorically it means morally pure, upright, blameless in heart and life, virtuous, holy. (1) Generally (Mark 6:20; Romans 7:12; 1 Corinthians 7:34; Ephesians 1:4; Ephesians 5:27; 1 Peter 1:16; Sept.: Leviticus 11:44). (2) Spoken of those who are purified and sanctified by the influences of the Spirit. This is assumed of all who profess the Christian name, hence hágios, saint, hágioi, saints, Christians (Acts 9:13-14, Acts 9:32, Acts 9:41; Acts 26:10; Romans 1:7; Romans 8:27; 1 Thessalonians 3:13). Spoken of those who are to be in any way included in the Christian community (1Corinthians 7:14). Holy kiss means the sacred Christian kiss, the pledge of Christian affection (Romans 16:16; 1 Corinthians 16:20; 2 Corinthians 13:12).
(II) Consecrated, devoted, sacred, holy, meaning set apart from a common to a sacred use; spoken of places, temples, cities, the priesthood, men (Matthew 4:5; Matthew 7:6; Matthew 24:15; Matthew 27:53; Acts 6:13; Acts 7:33; Romans 11:16, of firstfruit); of a male opening the womb (Luke 2:23); of apostles (Ephesians 3:5); of prophets (Luke 1:70; Acts 3:21; 2 Peter 1:21); of angels (Matthew 25:31).
(III) Holy, hallowed, worthy of reverence and veneration:
(A) Of God (John 17:11; Revelation 4:8; Revelation 6:10; Sept.: Isaiah 5:16; Isaiah 6:3).
(B) Of His Name (Luke 1:49; Sept.: Leviticus 22:2).
(C) Of the Holy Spirit (Matthew 1:18).
(D) Of holy covenant (Luke 1:72).
(E) Of the Holy Scriptures (Romans 1:2; Sept.: Daniel 11:28, Daniel 11:30).
The hiereús (G2409), priest, although he may not always be hágios, holy, performs priestly duties or ordinances and is sacred or consecrated (hósios [G3741]).
Deriv.: hagiázō (G37), to sanctify; hagiótēs (G41), holiness; hagiōsúnē (G42), holiness, the quality of sanctification.
Syn.: hieroprepḗs (G2412), a fitting sanctity; eusebḗs (G2152), godly, pious; hósios (G3741), pure from evil contact, ceremonially pure; áspilos (G784), without spot; hierós (G2413), sacred, outwardly associated with God; eilikrinḗs (G1506), sincere, pure.
Ant.: koinós (G2839), common, defiled; akáthartos (G169), unclean.
What is holiness? Like a coin, it has two sides. What we are, and what we do. We who are saved are already holy. Like the thief on the cross, from the first moment of salvation we are holy. We are God’s. We are separated from the world. We are consecrated to His purpose. When we are saved, holiness is an immutable characteristic. We are holy because by the definition of salvation God has separated us from the world and made us His own. Although when you think of holiness, purity comes to mind, and that is part of it, the first characteristic of holiness is that the item has been set apart from other items. It has a special use. You don’t wear your wedding gown to the office – it’s used only on your wedding day.
And Moses went up to God, and Jehovah called to him out of the mountain, saying, You shall say to the house of Jacob, and tell the sons of Israel: You have seen what I did to the Egyptians, and I bore you on eagles’ wings and brought you to Myself. And now if you will obey My voice indeed, and keep My covenant, then you shall be a peculiar treasure to Me above all the nations; for all the earth is Mine. And you shall be to Me a kingdom of priests and a holy nation. These are the words which you shall speak to the sons of Israel. (Exodus 19:3-6)
Holiness is also a practice and a lifestyle. While you are wearing your wedding clothes, you have a special sense of the importance of the day. It’s no ordinary day, it is filled with purpose. You are doing something important and life-changing. It’s a day to remember and to cherish. It’s a joyful day. And the “holiness” of your wedding clothes emphasize that. We are called to holiness, to put it on as a garment and to conduct ourselves as though we are holy.
Therefore girding up the loins of your mind, being sober, perfectly hope for the grace being brought to you at the revelation of Jesus Christ, as obedient children, not fashioning yourselves according to the former lusts in your ignorance, but according to the Holy One who has called you, you also become holy in all conduct, because it is written, “Be holy, for I am holy.” (1 Peter 1:13-16)
When you consider yourself holy, your behavior changes. It’s not so much a matter of holding yourself to a higher standard, it’s that the idea of doing some things just becomes inconceivable to you. And the more you live in a state of holiness, living before the face of God, or “coram deo“, the more ridiculous it seems to sin. This ongoing sanctification is the pursuit of holiness, and it begins by knowing who God is, who we are, and how He is changing us.
Follow peace with all, and holiness, without which no one shall see the Lord; (Hebrews 12:14)