Lust is sustained desire for a thought, person or object.
(Luke 15:16 | Luke 22:15 | Matthew 17:37)
The Nature of Lust:
You may object: "How can something that goes on in the privacy of my own mind be a sin? Who is harmed?"
Lust is a sin against:
Others -- it dishonors living souls by treating them as something to be consumed
God -- it scorns God's holy design and breeds the opposite of worship: ingratitude