There are a lot of definitions of idolatry, but I think all of them can be distilled into this one: worshiping the Created instead of the Creator; to see a partial aspect of reality as the whole of it; and to see the false as true. All three are related. The Created is a partial aspect of the whole, and because it is not "ultimately" true it is in a sense "false." (By the way, the word "whole" comes from the same root word as "holy" and "healthy.")
The Greeks has a similar concept with the "daimonic" (which was perverted into English as "demonic") which was when a part of you took over all of you. Take drug addiction, for an example.
The Commandment dealing with idolatry reads: "You shall have no other gods before me. You shall not make for yourself a graven image, or any likeness of anything that is in heaven above, or that is in the earth beneath, or that is in the water under the earth. You shall not bow down to them or serve them; for I the Lord your God am a jealous God, visiting the iniquity of the fathers upon the children to the third and the fourth generation of those who hate me, but showing steadfast love to thousands of those who love me and keep my commandments."
Pretty baroque language, in a King James kind of way, even to the point of extreme severity. But then, it is several thousand years old. If you look at it as good practical advice instead of "religion," it makes a great deal of sense.
You don't even have to use the word "God." You can do like philosophers do and use "the Absolute." Use "Ultimate Truth," or "Natural Law" or, like C.S. Lewis in The Abolition of Man, "the Tao."
Read the Commandment as "Lead your life in accordance with Truth, or bad things will happen." Sorta like jumping out of an airplane without a parachute and expecting to float to ground.
If you want to update it a little bit, and make it a more philosophical, you can read "You shall have no other gods before me" as "You shall believe in nothing but the truth." (The whole truth. Because if people don't, unholy - unhealthy - things usually happen. And we have about four thousand years of recorded history as to what the Truth is.)
Some current modern "other gods" - idols - have been such things as the State, country, the flag, Communism, science, the Earth, animals, the free market, Nature, and Man. All of them are the Created instead of the Creator, partial aspects of the whole, and in that sense are "false." And when people believe in them ("worship" them) bad things automatically happen ("visiting the iniquity of the fathers upon the children to the third and fourth generation of those who hate me...")
Visiting iniquity upon the children" surely sounds like a comment that when people do bad things it reverberates throughout the generations. Which, of course, it does. We're still paying for World War I.
I think it's pretty obvious that the worship of that false idol known as the State has, in the 20th Century, had some very bad effects. The historians I'm familiar with have settled on the number of dead as 177 million, although I've seen estimates of up to 200 million. That's "the iniquity visited upon the children" when people worship the False instead of the True. The Devil instead of God, if you want.
Communism, Nazism and fascism all were about the worship of the State. And all came out of Europe, too (Ray Bradbury has written, "If it comes out of Europe, it's probably wrong."). Currently, many European countries still worship the State (after two world wars, why haven't they earned their lesson?). And someday, fairly soon, bad things will happen to them for letting Muslims into their countries. I hope we don't save their keesters this time ("Fool me once, shame on you; fool me twice, shame on me." What saying covers the third time?)
All three of those idolatrous philosophies were about the worship of Man. Communism, especially. If you'll read some early Communist writers you'll find they actually thought Man could become God on Earth. Of course, Communism is officially materialistic and atheistic - and now it's pretty much became the State religion of the United States. Liberalism, you know.
And since the U.S.is starting to believe more and more in the State and the Good Things it supposedly can do, what we will get is Bad Things will happen to us (we have the whole of history before us. Why haven't we learned its lessons?).
Then we have the saying everyone knows: "God and Country." Well, I dunno. Nearly all countries claim God smiles upon them and frowns upon their enemies (I've always found it amusing when athletes, on TV, thank God or Jesus for their team's win. Someday I expect one guy on the losing team to burst into tears and say, "It's true! God hates us! Jesus hates me! He deflected my pass!")
The Commandment, "Do not use God's name is vain" doesn't have anything to do with saying naughty words. The correct translation is "Do not carry God's name in vain causes." Like "God and Country." When soldiers marched off to war by the hundreds of thousands, to foreign countries that weren't a threat to us, to die for "God and Country," that is a vain cause indeed.) Then of course they make the "ultimate sacrifice"...to enrich Cosmodemonic Transnational Megacorporations - more idols.
I consider myself a patriot, but I don't see how "pledging allegiance" to a flag (and isn't that a "graven image"?) has anything to do with patriotism.
The Nature-as-God environmental movement, with its worship of Nature, the Earth and animals (and humans as a cancer), is worshiping idols. Bad things will come from it. The philodoxer Pete Singer, the nutcase founder of the modern animal-rights movement, believes in infanticide, euthanasia and bestiality (specifically, sex with the higher primates. Someday, I'm going to write a satirical article about him and call it, "Your Monkey Wife, But Not Mine.")
If you want to describe the extreme environmental movement in one sentence, it's the worship of "anything that is in heaven above, or that is in the earth beneath, or that is in the water under the earth."
Even science can be an idol. I used to watch the late Carl Sagan on TV looking up at "anything that is in heaven above" with a breathless, almost religious look on his face. He was, not surprisingly, an atheist and a socialist. Science was his religion. He never realized (and would never believe it) that he was worshiping an idol.
Most modern science believes the material universe is all that there is. Life is just an epiphenomena. It's a dead universe, gigantic beyond human comprehension, with a few infinitesimal and maybe temporary sparks of life. The religious view (and the view of much Idealistic philosophy) is that the truth is the exact opposite. The material universe (the Created) is the infinitesimal part. It's just a teeny-tiny itty-bitty little part of the Creator, like the surface of an ocean.
When you have science based on philosophical materialism, it believes only that the Created exists, that the part is the whole. It believes in an idol. And bad things will come from this. That's why it never surprised me when I found out the Nazis were strongly influenced by evolutionary theory (and evolutionary theory is indeed just an infinitesimal and in many ways amusing explanation for a tiny part of that Whole).
When I see Muslims, Jews and Christians fighting over a sliver of land in the Middle East, I wonder, "Do they really think God lives in a piece of dirt?" Or in a house, like a church? Or in a piece of cloth, like the clothes someone wears? Or in a flag? (I learned a new word not that long ago: "revanche." It means "the policy of a state intent on regaining areas of its original territory that have been lost to other states." The root word is "revenge.")
The human race has a decided, and thoroughly unfortunate, tendency to ignore the lessons of the past - to see them as primitive superstitions. It's especially bad in the "elites," who often have no ears to hear, no eyes to see, ditches they rarely avoid, and hand-baskets-to-Hell they're always falling into. It is, to say the least, an unhealthy tendency. You can even all it unholy. It certainly leads to unpleasant things, even - or is it most especially? - for the innocent.