In Webster's dictionary lust is defined as:
a strong feeling of sexual desire OR a strong desire for something.
Is this is the same as the Greek and Hebrew definitions? If so, is lust always sin or is lust sometimes acceptable?
Yes, in a simple form and a more complicated one.
I do not often like to refer to the Greek (mostly because I know little about Greek) but in this case I think we need to.
One word in the original language which you are probably referring to as 'lust' is this. Greek word
By looking at the various tenses of the word in different contexts we can probably be in a position to answer the question more objectively.
First, using the ESV passions (in each case this seems to be used with the implication of evil passions:
Next desires (this is mostly used for evil desires but sometimes just neutral desire of even a holy one:
The ESV translates this word using 'lusts' in only a few instances all related to strong sexual desire outside of marriage. Other English translations often use the previous 'desires' as lusts also
Complete listing in ESV version:
Sometimes it is translated as covet (basically a discontented desire)
Lust is a term when denoting an evil impulse assumes a desire not derived out of love for God. As the bible defines good behavior as deep love for God from which all other desires should stem, lust is explained by a strong contrary desire to love for God.
There is deeper doctrinal aspect to the Biblical explanation of this: Humans are born dead with evil desires as punitive justice for the sin of Adam extended to all. By faith in Christ they are regenerated internally from sin and death to love and life. Having new life and thus new love for God as a result of being declared righteous by grace apart from moral deeds, they have a dual nature. Consequentially a believer has strong holy desires as well as remaining sinful desires of the old life inherited through Adam. This indwelling sinful nature lusts against the new nature and they are at war with each other. Therefore lust is a remnant of any desire stemming from the old nature of Adam under the curse of God's law for his sin imputed to all. Any desire from that nature is the lust of the flesh. It is sinful because human nature is contrary to God's nature under His punitive justice. It knows nothing of love for God. Being a Christian is first obtaining new inward life in Christ and inwardly dying to the 'fleshly' life inherited from Adam. Then upon the death of the body, the entire evil nature inherited from the curse of Adam's sin will be gone, along with all its desires forever.
Catholic perspective: always wrong/bad, not always sinful.
Pertinent numbers in the Catechism of the Catholic Church:
2514 St. John distinguishes three kinds of covetousness or concupiscence: lust of the flesh, lust of the eyes, and pride of life. [cf. 1 Jn 2:16]. In the Catholic catechetical tradition, the ninth commandment forbids carnal concupiscence; the tenth forbids coveting another's goods.
2515 Etymologically, "concupiscence" can refer to any intense form of human desire. Christian theology has given it a particular meaning: the movement of the sensitive appetite contrary to the operation of the human reason. The apostle St. Paul identifies it with the rebellion of the "flesh" against the "spirit." [cf. Gal 5:16,17,24; Eph 2:3]. Concupiscence stems from the disobedience of the first sin. It unsettles man's moral faculties and, without being in itself an offense, inclines man to commit sins. [cf. Gen 3:11; Council of Trent: DS 1515].
From a catholic perspective, therefore, lust is always wrong/bad. Whether it is sinful depends on full knowledge and full will, for Catholics, a matter for the confessional.
(E.g., you, a person, and another passes before you dressed to appeal to the lower (now rebellious after original sin) instincts. Those instincts spontaneously combusts. One does not commit a sin if one, saying a prayer, averts one eyes and rejects those base instincts. But as said above, for Catholics, the judgment belongs to the confessional.)
@Mike has a great answer, but I think I'll expand on the sexual aspect of it. Why is sexual lust a sin?
Because desiring someone other than your spouse is contrary to God's eternal committal nature. The relationship between a husband and wife is suppose to reflect the relationship between Christ and the church.
The very definition of sin is "to miss the mark", and since the mark is Christ, and to lust is to do something Christ wouldn't do, to lust is to sin.
It would be a sin for the church to desire someone other than Christ, and it would be a sin for Christ to desire someone other than his church, his bride. To want someone sexually without committing to them for life is to want them for selfish reasons. Christ loves unselfishly.
The true definition of lust is to have such an intense or strong desire that it causes one to violate the rights of others. Lack of self control and reason. It is never a sin to have a strong desire to do good things in life, like making the world a better place. Or having desires to serve others in God's name. It is when one places their desires in a position where it causes one not to put God first, or with his children. And causes one to lose all control of his or her senses. Like love. Love has the same effect as cocaine. Where if the person loses love of another, they don't want to eat or sleep. Or they defy or disrepect their parents for sake of loving another person. This is lust. It is mention in the bible that for the LOVE of money is the root of all evil. That means a person places money higher than God, themselves or others. There is no sin for wanting money and having sensible control with money.
whether it's about desiring sex with someone who is not one's spouse, or whether it's about sexually objectifying any person of the opposite gender (or the same gender if that's one's sexual orientation), attitudes behind that is sin. appreciation of sexual beauty need not be this objectification. desiring to have sexual fulfillment need not be this objectification.
looking at it as a heterosexual male, i would say that the sin is when, in a man's mind, women are toys to be played with. but looking forward to sex with one's spouse (or soon-to-be-spouse) is normal. not sin.