How defined is the threshold of graveness for mortal sins? (Catholicism)
As with such matters of conscience, there will always exist some wiggle room in the definition here.
Interesting question, as threshold as to what constitutes grave matter is much more than simply listing a number of actions that constitutes as mortal sin. There may be extenuating circumstances that may diminish the nature of a particular action that would make it normally a serious sin. Circumstances could theoretically in some situations, mitigate any culpability all together.
For example, a person who does not have the full use of their reason can not sin seriously.
Not every violation of the Ten Commandments constitutes a mortal sin. For instance, a 6 year-old child stealing a nickel from a rich man might violate the commandment to "not steal", but it is not mortal, because it is not grave matter.
There are examples of such stories, even in the lives of some of the saints.
The Catechism of the Catholic Church defines grave mater as follows:
1858 Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man: "Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and your mother." The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger.
We must also remember that the Ten Commandments is not an exhaustive list of mortal sins.
So, while it is almost impossible to come up with a definitive list of sins that are mortal, here are some that should be confessed (pardon the randomness, I did it off the top of my head and from a few other lists):
Murder
Blasphemy
Idolatry
Adultery
Pride
Abortion
Rape
Despair in God's mercy
Occult activity
Superstition
Divination / Using Magic
Illegal Drug Use
Intentionally getting drunk
Defiant disobedience of your parents, for non-adult children
Wanton destruction on another's property
Lustful thoughts that are dwelt upon
Prostitution
Fornication
Pornography / Masturbation
Homosexual Acts
Incest
Theft
Greed
Envy
Abuse of the Poor / not giving to the poor
Defrauding others of what they are due
Serious Lies / Lying under oath (perjury)
Jealousy
Laziness
Bad-mouthing others / gossip
Missing Mass on Sunday or a Holy Day
Presumption of God's forgiveness
Anger that has no justification
Hatred of others
Euthanasia
Grave scandal
Heresy
Apostasy / Schism
Gluttony
Extortion
Terrorism
Divorce and re-marriage
Spiritual Sloth
Source
Often we have questions about the seriousness of our action and doubts arise as to the seriousness of something we have done. Thank God, we as Catholics have the sacrament of confession. In such situations, it is best to humbly make a good confession and explain the situation to a priest, who is trained to deal in this matter.
In the moral theology of Catholicism, a mortal sin requires that all of the following conditions are met:
Its subject matter must be grave. (The term "grave sin" is used at times to indicate grave matter, and at times to indicate mortal sin. But it always remains true that the following two conditions are requisite for mortal sin.)
It must be committed with full knowledge (and awareness) of the sinful action and the gravity of the offence.
It must be committed with deliberate and complete consent.
With respect to a person's full knowledge of a certain act being a grave sin, the Catholic Church teaches that "unintentional ignorance can diminish or even remove the imputability of a grave offense. But no one is deemed to be ignorant of the principles of the moral law, which are written in the conscience of every man. The promptings of feelings and passions can also diminish the voluntary and free character of the offense, as can external pressures or pathological disorders (mental illness). Sin committed through malice, by deliberate choice of evil, is the gravest." Furthermore, Catholic teaching also holds that "imputability and responsibility for an action can be diminished or even nullified by ignorance, inadvertence, duress, fear, habit, inordinate attachments, and other psychological or social factors." In this regard, a sin committed while one is inebriated may lack the awareness and consent necessary for the sin to be mortal. But when one becomes aware of the danger of excessive drink, such drinking itself becomes a serious matter.
Grave sins should not to be confused with the seven deadly sins, which are so called because they lead to other sins; they are not necessarily grave sins.
Mortal sins are called "grave sins" under the Code of Canon Law due to the "grave" nature of all mortal sins, and the terms are used there interchangeably. This does not deny the distinction given above, that there may be grave matter but not a grave sin if the other conditions of knowledge and freedom are not present.
Mortal sins must be confessed by naming the specific offence along with how many times it was committed. Mention of how long since one's last confession is to establish whether one is truly penitent – has a purpose of amendment.
It is not necessary to confess venial sins although they may be confessed, a practice that began with the Irish monks around the 12th century. Venial sins are all sins that are not mortal. The Church encourages frequent, intelligent use of the sacrament of confession even if a person has only venial sins, in view of the benefits that might be derived. Mortal Sin (Catholic Encyclopaedia)