Traditionally thought of as lawlessness--a transgression of a moral law. It has also been used in terms of an offense or violation.
When mankind originally set forth on his own, he became separated from the source, and in the process, forgot that it was his own idea. The separation is painful (illusion of separation) and man wondered, "What horrible thing did I do to get punished like this?" This led to ideas such as original sin, toxic shame, etc. Consequently, man is searching for that lost part of himself. However, the act of love creates a temporary bridge across the void, giving us a taste of what the true Union is like.
To sin is to miss the mark, because we are not perfect, and is created by fear. Deception and denial are elements that allow fear to manifest. The emptiness and frustration produced by fear must be replaced by forgiveness. Any negative thought, action, word or deed, places a debt on the individual that metaphysically we call Karma. The debt may be repaid within a lifetime or subsequent lifetimes. When the debt through our atonement is complete, the experience can be shared by all. When we fail to atone, we miss the mark.
It is possible to sin even in the world of spirit. The sins of the spirit world are the sins of selfishness, but they are speedily revealed there. They are known as soon as the thought is in the mind, and the effect is seen far more quickly than it is in the world of matter. It registers on the one who commits the sin and makes him spiritually lower than he was before. Although it is difficult to express in words what these sins are, the closest word is that of selfishness.