Simply put, sin= DENIAL of REALITY.
Many people are totally confused about the meaning of sin, and that is because they are totally confused about God, Life, Jesus, Creation etc. On one hand they talk about sin and on the other about "free will" and they can not reconcile the two, which causes real confusion in their minds and their lives. In order to understand sin one must know basic things about God and Creation and then they will understand what sin is. It is like this:
The NATURE of God is Love, Truth, Joy, Mercy, Kindness, Goodness etc but the NATURE of God is NOT hate, malice, jealousy, meanness. The Nature of God is the REALITY of the WHOLE of Creation. The Creation is divided into three realities

ure spirit, semi-spirit and physical. The physical reality, in which we exist, is a time-space, EVOLUTIONARY and EXPERIENTIAL reality. We, humans, and many other intelligences created in this physical reality, start our ascent towards God through trials and tribulations of life, learning along the way the Truth of what God is.
Because this physical reality is EVOLUTIONARY and EXPERIENTIAL , error and truth exist side by side as a CONTRAST to each other. Error, however, not being a true nature of God, is actually NOT real even though it does seem very real to us. Error, hate, malice, meanness, jealousy etc are "real illusions" in this time-space, physical creation.
Ours is an evolutionaty and experiential ascend toward God. Every step along the way we are faced with decisions. We notice that EVERY SINGLE TIME we embrace the Truth that God is ( Love, Kindness, Goodness) we GROW and EVOLVE, while the embrace of hate, malice, meanness brings decay and retardation.
This is all EVIDENT if you just observe and discern life !!
Embracing God's qualities brings satisfaction and happiness because that is the True Nature of Reality while embracing the "real illusion" of hate, malice etc brings disappointment and emptiness because they are NOT REAL. Here, then, is the answer to what sin is.
By using your God given "free will" any SANE mind should be able to see and discern what works (God) and what doesn't ( illusion ) and then CHOOSE GOD. However, any SANE mind(?) that CONSCIOUSLY and INTENTIONALY CHOOSES evil, hate and malice and deliberatelly embraces those actions which bring decay and death, chooses to work against God=SIN.
Cheers