Grave matter is a term used as a criterion for mortal sin (the others being knowledge and free will). An authoritative definition of mortal sin is provided by St. Augustine as “something said, done, or desired contrary to the eternal law, or a thought, word, or deed contrary to the eternal law” as a voluntary act.
A sin being a “voluntary” act is critical to the application of graveness: an involuntary sin connotes material sin, and voluntary sin is a formal sin. The graveness of an act is more or less its degree or lightness. This determination on the constitution of an act as a grave matter involves various complexities:
Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man: “Do not murder, 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 (CCC 1858).
By this, the Church teaches that some acts are always grave, while others involve a degree of lightness.