Worship
Not the songs before the sermon, but the whole self answering God
In ordinary speech the word has shrunk almost to nothing. "Worship" now usually means the music at the front of a Sunday service, the part before the sermon. The Bible means something incomparably larger. Worship is the response of the whole self to the worth of God. The old English word is a clue: worship is worth-ship, the act of ascribing worth, of recognizing what something is truly worth and answering accordingly. And it is not a rare or optional act for the especially religious. Everyone does it. Every human heart pours out its highest allegiance and adoration onto something. The only question any of us actually faces is not whether we will worship, but what.
Worth-ship: telling the truth about God
At its root, to worship God is simply to tell the truth about Him and answer it with your whole being. The Psalms call it ascribing to God what is already His: "Give unto the LORD the glory due unto his name; worship the LORD in the beauty of holiness" (Psalm 29:2). In John's vision of heaven the elders fall down and say it plainly, "Thou art worthy, O Lord, to receive glory and honour and power" (Revelation 4:11). Worship is not flattery and it is not God needing applause. It is a creature finally seeing reality straight: God is infinitely worthy, and to withhold worship is to live a lie about who He is and who we are. To give it is simply to come into the truth.
In spirit and in truth
The defining word on worship fell in an unlikely place, said to a Samaritan woman at a well who wanted to argue about the right mountain. Jesus lifted the whole question off of locations and onto something deeper. He told her the hour had come when true worshippers would worship the Father not in this place or that, but in spirit and in truth, and then He gave the reason: "God is a Spirit: and they that worship him must worship him in spirit and in truth" (John 4:24). Both words carry weight. In spirit: worship is first an inward thing, the heart engaged, not lips moving over a cold heart. In truth: it must be aimed at God as He actually is, not at a flattering invention. Sincerity aimed at a false god is not worship, and correct doctrine with a dead heart is not worship either. The real thing needs both: a true heart, toward the true God.
Not mainly music, and not mainly Sunday
Because worship is the whole self answering God, it cannot be confined to singing or to a single morning a week. Paul's great definition of it never mentions music at all: "present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service" (Romans 12:1). The body, the ordinary, the Monday, offered up to God, that is worship. Which is why nothing is too small to count: "whether therefore ye eat, or drink, or whatsoever ye do, do all to the glory of God" (1 Corinthians 10:31). The gathered church and its songs matter enormously, but they are meant to be the concentrated expression of a worship that fills the rest of the week, not a substitute for it. Worship is not finally an event you attend. It is a life you hand over.
Everyone worships something
This is why the Bible is so relentless about idolatry, and why idolatry is not a quaint problem of people bowing to statues. The human heart is built to adore, and if it does not adore God it will adore something else and call it by another name. Paul names the exact swap at the root of all our trouble: people "worshipped and served the creature more than the Creator" (Romans 1:25). An idol is just a good thing, money, romance, success, approval, security, a child, even a cause, handed the worth-ship that belongs to God alone. Everyone is bowing to something. The modern person who worships nothing religious still organizes a whole life around something they cannot bear to lose. The question worship asks is simply: is the thing you have built your life around actually worthy of it (see the heart and what it serves)?
Worship is the very first thing God claims from us. The commandments open not with morality but with allegiance: "Thou shalt have no other gods before me" (Exodus 20:3), and when the devil offered Jesus the whole world for one act of worship, He refused with the same law: "Thou shalt worship the Lord thy God, and him only shalt thou serve" (Matthew 4:10). And it is the last thing too. The final pictures of heaven are not of harps and clouds but of unending worship, a whole creation crying, "Worthy is the Lamb that was slain" (Revelation 5:12). Whatever you will be doing forever, you might as well begin learning now.
Where this lands
It helps to know one more thing: we tend to become like what we worship. Give your deepest adoration to money and you grow grasping; give it to image and you grow hollow; give it to God and you slowly grow like Him. That is not a threat but the kindest of invitations, because there is only One who is wholly worth becoming like (see who He is). So worship is not God extracting tribute from reluctant subjects. It is the Maker calling His creatures back to the only thing big enough to hold the whole weight of a human heart, and setting them free in the process. You were made to worship. The good news is that there is Someone entirely worthy of it.
Related: The Character of God, The Fear of the Lord, the Heart, Prayer, and Communion. Scripture from the King James Version, linked to BibleHub; the words of God are marked in gold, the words of Christ in purple.