According to Roman Catholicism, a venial sin (meaning "forgivable" sin) is a lesser sin that does not result in a complete separation from God and eternal damnation in Hell as an unrepented mortal sin would. A venial sin involves a "partial loss of grace" from God. They do not break ones friendship with God, but injure it.
An act, when it is not ordered towards that which is good, is considered to be sinful – either venially or mortally. When such an act is venially sinful, it entails subject-matter that is not considered to be "grave." Such an action, even if it is committed with full knowledge and full consent, remains venial, so long as the subject-matter of the act is not serious. If the subject-matter of a given act is "grave," however, the commission of that act may be mortally sinful. Thus, in discussing the distinction between venial and mortal sin in his Summa Theologica, St. Thomas Aquinas indicated that a venial sin differs from a mortal sin, in the same way that something imperfect differs from something that is perfect.
As such, one can arrive at what kind of sin, for example, was committed, by asking the following three questions:
- Did the act involve grave matter?
- Was the act done with full knowledge of the grave and sinful nature of the act?
- Was the act done with full consent of the will?
If all three questions are answered in the affirmative, the criteria for a mortal sin have been met. If any one of the three questions are answered in the negative, only the criteria for a venial sin have been met. In cases of doubt regarding any of these three questions, it is assumed that the criteria for a mortal sin were not met.
Each venial sin that one commits adds to the penance that one must do. Penance left undone during life converts to punishment in Purgatory. A venial sin can be left unconfessed so long as there is some purpose of amendment. One receives from the sacrament of reconciliation the grace to help overcome venial, as well as mortal sins. It is recommended that confession of venial sins be made. Venial sins require some kind of penance.
According to the Magisterium, venial sins usually remain venial no matter how many one commits. They cannot "add up" to collectively constitute a mortal sin, but their accumulation does lead to being more vulnerable to committing mortal sin. There are cases where repeat offenses may become a grave matter. For instance, if one were to steal small amounts of property from a particular person, over time one would have stolen enough that it would develop into a serious theft from that person.
In all this, one ought not to take venial sin lightly, especially when committed deliberately. No one without a special grace (according to the Magisterium, given only to the Blessed Virgin Mary,) can avoid even semi-deliberate venial sins entirely (according to the definition of Trent). But one must, to avoid mortal sins, seek (as far as possible) to overcome venial sins. The Magisterium teaches that although a number of venial sins do not themselves add up to a mortal sin, each venial sin weakens the will further, and the more willing one becomes in allowing such falls, the more one is inclined towards, and will inevitably fall into (if one continues along this path), mortal sin.
- "Catechism of the Catholic Church".
1863. Venial sin does not deprive the sinner of sanctifying grace, friendship with God, charity, and consequently eternal happiness.
- "Venial Sin". Dictionary.com.
- Donovan, Colin. "Mortal verses Venial Sin". EWTN.
- Saunders, Fr. William (March 30, 1995). "The Arlington Catholic Herald".
- Code of Canon Law (CIC 988).
- "The Necessity of the Sacrament of Penance". Catechism of the Council of Trent.
These words were said by our Lord in reference to grievous and mortal sins, although at the same time lighter sins, which are called venial, also require some sort of penance.
- "However venial sin does not break the covenant with God. "
- " But do not despise these sins which we call light: if you take them for light when you weigh them, tremble when you count them. A number of light objects makes a great mass" 
- Rev. Donald F. Miller, C.Ss.R., 'Examen for Laymen' http://www.catholicculture.org/culture/library/view.cfm?id=3635&repos=1&subrepos=0&searchid=753266 Retrieved 2011-05-28.
- "The most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Savior of the human race, preserved immune from all stain of original sin. From the first instant of her conception, she was totally preserved from the stain of original sin and she remained pure from all personal sin throughout her life. " 
- "Deliberate and unrepented venial sin disposes us little by little to commit mortal sin."
- Sin in Catholic Encyclopedia
- The Venial Sin by Honore de Balzac
- Catechism of the Catholic Church – Mortal and Venial Sin