From Saint Wiki
Calumny and Slander. Calumny is, correctly speaking, a false and injurious charge against another, such as imputing to him habits that he does not possess, or sins which he has not committed. Slander consists in spreading or exaggerating evil reports, unjustly tending to injure our neighbor's reputation; detraction is the making known, without just cause, the faults of another's character. Slander and calumny are, therefore, the most pernicious of lies, because they falsely ruin another's good name; and unless excusable, from ignorance or inadvertence, and other extenuating circumstances, are serious, and may be mortal sins.