I am assuming that you don't mean what we mean by lust, but rather you are questioning what the biblical term translated into English as lust means. Well, here are some examples that make fairly clear what lust is. These are most of the verses found in the bible which are translated as lust. I got bored and left some of the later old testament ones out because they felt repetitive:

Numbers 15:39
You will have these tassels to look at and so you will remember all the commands of the LORD, that you may obey them and not prostitute yourselves by going after the lusts of your own hearts and eyes.

Job 31:1
"I made a covenant with my eyes not to look lustfully at a girl.

Proverbs 6:25
Do not lust in your heart after her beauty or let her captivate you with her eyes

Jeremiah 5:8
They are well-fed, lusty stallions, each neighing for another man's wife.

Jeremiah 13:27
your adulteries and lustful neighings, your shameless prostitution! I have seen your detestable acts on the hills and in the fields. Woe to you, O Jerusalem! How long will you be unclean?"

Ezekiel 16:26
You engaged in prostitution with the Egyptians, your lustful neighbors, and provoked me to anger with your increasing promiscuity.

Ezekiel 20:24
because they had not obeyed my laws but had rejected my decrees and desecrated my Sabbaths, and their eyes lusted after their fathers' idols.

Matthew 5:28
But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart.

Romans 1:26
Because of this, God gave them over to shameful lusts. Even their women exchanged natural relations for unnatural ones.

Romans 1:27
In the same way the men also abandoned natural relations with women and were inflamed with lust for one another. Men committed indecent acts with other men, and received in themselves the due penalty for their perversion.


Edited: removed doubles