The Catechism in Questions & Answers

357. What does the Eighth Commandment mean for us today?

Today the meaning of the Eighth Commandment transcends the original prohibition against untruthful speech and actions. White lies, half-truths, and statements intended to conceal the true facts, as well as slander, are thus also violations of the Eighth Commandment. Bragging and exaggeration, duplicity and hypocrisy, spreading rumours, defamation and flattery are likewise expressions of untruthfulness.
Everyone is called upon to strive for sincerity and truthfulness. Our conduct in society and in business should also be oriented by the Eighth Commandment.

Untrue assertions about another person which do him harm, do injury to his honour, or offend him are described as defamation or slander.