The Catechism in Questions & Answers

482. What is the meaning of “original sin”?

“Original sin” is the condition of separation from God (remoteness from God) that came into being through the fall into sin. Since the fall into sin, sin has weighed upon all human beings (cf. Genesis 3: 20; Psalm 51: 7; Romans 5: 12, 18-19). Every human being is thus a sinner even before he is capable of acting or thinking.

The fall into sin: see Question 88. et seq.