A classical definition of sin might be that it's some kind of attack you perform which is "against the rules." Some breaking of the law or some unholy evil act which is disallowed and undermines everything that is good. In A Course in Miracles, the definition of sin is a lot broader than this. For example:

"The world of bodies is the world of sin, for only if there were a body is sin possible."

This suggests that the whole world is made of sin. And this isn't just a speculation, because ACIM goes on to say that physical matter is in fact a "granite block of sin" masquerading as something solid.

"The body's eyes see ONLY form. They cannot see BEYOND what they were MADE to see. And they were made to look on error, and NOT see past it. Theirs is indeed a strange perception, for they can see ONLY illusions, unable to look beyond the granite block of sin, and stopping at the outside FORM of nothing. To this distorted form of vision, the OUTSIDE of everything, the wall that stands between you and the truth, is wholly true."

Here we're told that the body's physical sight can only see error and not see past it. An error which is not "overlooked", is not forgiven, and so it is regarded as sin, demanding punishment. Everything that the physical sight sees is an illusion, therefore, because the physical sight is completely unforgiving and cannot transcend the illusion to reach the truth. These illusions - really a reference to anything the body can see - therefore all things physical - is called "the granite block of sin." Jesus uses the term "granite" symbolically because this is a very hard form of physical material.

"The body is the ego's idol; the belief in sin made flesh, and then projected outward. This produces what SEEMS to be a wall of flesh AROUND the mind, keeping it prisoner in a tiny spot of space and time, beholden unto death, and given but an instant in which to sigh and grieve and die in honor of its master."

So now the physical body, which is part of the physical environment, is made by projecting the idea of sin outward and turning it into flesh, which symbolizes sin. Effectively everything that we regard as physical is made of the idea of sin, projected as physical matter. Physical matter comprises "outside surfaces" which you cannot see past with the physical eyes, and is really "nothing" pretending to be a solid block.

"Sin is the proclamation that attack is real, and guilt is JUSTIFIED."

Sin is also the idea of attack. If attack is real, then the world is real, because "the world was made as an attack on God." Being "the world of sin", the world of bodies was made as an attack on God. Its forms testify that God's son has been attacked. That is, "the world is a picture of the crucifixion of God's son." So now all physical forms are a form of attack, or attempted attack, upon God and his son. And this is sin, because indeed if attack were possible, the world would be sinful.

"Sickness is but another name for sin."

Here Jesus goes further to include that any form of sickness is also the same thing as sin. Whether it be a headache or a broken body or a heart attack or a wound or severe death. These are all signs and proofs that sin is real. In fact, we even make the body sick to try to prove that our brother has sinned against us, per "brother at your hand I die", and the willingness to use the body as "a picture of crucifixion" to demonstrate that our brother is the one who caused our suffering.

"Sin is insanity."

Sin and insanity are also the same thing. Which means, the entire ego thought system, all mental illness, and all its physical expressions, is sin. Any belief in an opposite to God, any attack thought or grievance, all forms of sickness, all unforgiveness, it is all sin. This include all false perceptions, all physical perceptions, all body identification, all fixating on what things appear to be physically, and all blindness by perception of form, all testifies that sin is real. These are the beliefs of an insane mind which lacks vision and is unconscious.

This also ties in with the fact that sin is death, because "without the idea of death (sin) there is no world." "Sickness is but a little death", and since sickness is another form of sin, sin is death. This relates closely also to the fact that "the wages of sin is death", ie if you are in sin it logically follows that sin demands punishment and therefore you as a body must suffer and die in order to atone for your sin.

"They do not realize that, to deny God, is to deny their own identity, and in this sense, the wages of sin IS death. The sense is very literal; -- denial of Life perceives its opposite, as ALL forms of denial replace what IS, with what is NOT."

The denial of life perceives death, which is the opposite of life. Sin is the denial of life, and therefore induces death. All forms of death are forms of sin.

"If sin is real then punishment is just, and cannot be escaped."

"If sin is real, salvation must be pain."

"Pain is the cost of sin, and suffering can never be escaped if sin is real."

"If sin is real Its offering is death, and meted out in cruel form to match the vicious wishes in which sin is born."

Essentially, anything other than the truth of God, is the idea of sin. Just as it is also the idea of fear, and the idea of guilt, and the idea of sickness, and the idea of death. It is all the same single system of darkness, delusion and insanity. And WITHIN this system of insanity lives "the world", the unforgiven world, in which only the physical is regarded as real because "only the body makes the world seem real."

"The world is false perception."

When your vision is narrowed to see and focus only on the physical world or what the physical senses testify to, you are blinded by perception of form. And now you see only sin. Which means seeing any body, any physical sickness, any symptoms, any problems, any kind of suffering or unhappiness, any breakdowns or breakages, and dangers, any reasons to be afraid, this all means you've entered into a sinful perception and are now making sin real.

And this is the stuff of nightmares, because once you have closed your mind to the openness of innocence, you've decided only sin exists and cannot be escaped. When the mind is shutting out the light of truth and is becoming unforgiving, which means it is attacking and separating, other people will appear to be "only bodies", as if they are purely physical, with no component to them that is anything but physical. This is an imprisonment of yourself and them anchoring you both into the level of form and inescapable problems.

"For bodies cannot forgive."

To be in sin is to be insane, to perceive falsely, to give reality to physicality, to be unforgiving, to be in fear, to feel horrified and terrified in a nightmare, to regard problems as utterly unsolvable, to feel imprisoned and locked in with no hope, to believe all sickness is real and permanent, to forget to laugh, and to hate yourself.

There HAS to be more than the physical in order for there to be innocence. There HAS to be more than bodies, more than physical suffering, more than physical problems, and more than a physical world. There HAS to be a transcendence of all things physical if there is to be forgiveness, because to forgive is to overlook and NOT see illusions. Illusions not recognized are a false perception which is horribly blinding and upsetting. Illusions recognized dispels them and disregards the physical as nothing real at all.

"There is no sin", in ultimate truth, because "nothing real can be threatened." The immortal cannot be sick or suffer or die. God's creations are permanently perfect. There cannot be an attack and nothing can be in danger. Therefore, there cannot be a physical world, bodies cannot be real, and suffering cannot be true. Problems cannot really exist, and you cannot be alone.

Given that the world of bodies is the world of unforgiveness and sin, in which sin is not recognized as a mistake, recognizing the world as a mistake also recognizes that it is false. Mistakes can be undone and corrected. Forgiving the world is its undoing. First the veil of sin is lifted away so that we can recognize it is nothing but an illusion, we see through and past its illusion of being real and solid and physical, and see beyond it to the light of the real world. And in this light, the physical world, seen only in false perception, is gone forever.