“Sin” is a thought or action that is, at root, an offense against God. It’s a violation of the great commandment to love God above all else, and to love your neighbor as yourself.
Catholic Tradition and Scripture name two categories of sin: mortal sin and venial sin.
Mortal sin is serious enough that it kills the life of grace within us. Every act of mortal sin effectively refuses God’s offer of grace and life — with such an act, we choose death.
There are three conditions that make an act a mortal sin:
An act of grave matter that is…
Committed with full knowledge and…
All three conditions must be met for it to be a mortal sin. If one condition is seriously lacking, it’s not mortal — it’s considered a venial sin. (See Catechism section on mortal sin, 1854-1864).