Ever heard of the term "lust" before? Chances are the answer is yes. And chances are that nobody has given you a definition for the word "lust". And when it is defined, it's rarely defined according to what the Bible says it means. All it takes is a simple internet search to see that the Bible, while often used to explain the definition given, is not used as the source of the definition. In fact, if you read the articles in the link carefully, you'll notice that each source gives a slightly different definition of lust. And some of these sources are closer to the biblical definition than others.
So, which one is correct? As it turns out, none of the definitions given on the first page in the above-linked search results are correct. Some are close, but they all fall into the same trap: Namely, associating lust with strong, sexual desire. And I'm just talking about all of the Christian resources that appear in those search results. They're all wrong to some degree, and all in the same way!
To understand why they're all wrong, we first need to understand some of the verses used to support the given definition of lust. We'll start with Matthew 5:28:
But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart. [KJV]
It is admirable for people to try to get the definition from the biblical text. But often, the text is mistranslated or not read carefully. Or both. And in this case, it's both. Many Christians believe that merely looking at a woman is lusting after her, especially if she's wearing revealing or form-fitting cloths (or is entirely naked). But if we read the passage carefully, we'll notice that it's not looking at a woman that is lust. Instead, it's looking at her with lust, that's the problem. Her clothing (or sometimes lack thereof) is completely irrelevant to whether the man is lusting after her.
So that clears-up some of the misunderstanding of the text, but it doesn't give a solid definition of what lust is. It just tells you what lust is not. The KJV actually does a good job of providing a definition in Romans 7:7:
What shall we say then? Is the law sin? God forbid. Nay, I had not known sin, but by the law: for I had not known lust, except the law had said, Thou shalt not covet. [emphasis mine]
Lust is the same as coveting. And coveting is a strong desire. Notice that some of those search results listed above do give this definition, but with an emphasis on sexual desire, and stating that the desire is always sinful. Let's take a look at what the Greek words translated as "lust" and "covet" mean. The word in question is "epithymeo", which means "to desire" (usually a strong desire). The KJV and other translations translate this word as "desire", "covet", or "lust" based on the context. If the context is negative, it's translated as "lust" or "covet" depending on whether the translators believe the text is referring to a sexual desire. If the context is positive or neutral, then it's translated as "desire". But it's the same word.
By the way, the Law of God does not say "You shall not covet." What Paul is doing here, by quoting part of the commandment, is invoking the entire commandment which he is assuming the reader knows. Here's what the full commandment being invoked says:
You shall not covet your neighbor’s house. You shall not covet your neighbor’s wife, or his male or female servant, his ox or donkey, or anything that belongs to your neighbor. [Exodus 20:17; NIV]
As you can see here, the command is not, "do not covet, period", but rather gives a list of things that you are not allowed to covet. The things listed here are all things that you cannot legally obtain. Notice that there are human beings on this list. Namely, your neighbor's wife, and male and female servants.
And here's one of the misunderstandings about lust. Lust is not a strong sexual desire. Lust is when a man strongly-desires that a woman be his own wife. It can also be when a man strongly-desires having sexual relationships with a woman. So in this instance, part of the definition given is correct, especially if the source points out that the desire is a choice. But the desire is not simply to derive sexual pleasure just by looking upon her. The desire is to actually have sexual relationships with her. And this is one of the main points where nearly everyone who defines lust gets it wrong. That is not to say that the former is beneficial or good (it certainly is not, nor is it what God intended), it's just not what the word "lust" means.
Jesus' teachings against lust is actually him linking the 10th Commandment "you shall not covet your neighbor's wife" to the 7th Commandment "you shall not commit adultery". That is, you shall not desire that your neighbor's wife be your own wife, nor shall you desire to have sexual relationships with her. For if you do, then you have already committed adultery with her in your heart.
Note also that this means that one can lust after a woman who is not married to anyone and not be guilty of sin. In other words. Lust is only sinful in the context that acting upon it would be a case of adultery. If a man acts upon his lust of an unmarried woman, he, by definition, is not guilty of adultery and therefore, not guilty of adultery in his heart. If the woman is married, but the man does not know it, then when he finds out that she is married, he must stop desiring her immediately. Because if he continues to desire her, knowing the truth, he then becomes guilty of having committed adultery with her in his heart.