What is 'Zero-Knowledge Proof' completeness?

What is 'Zero-Knowledge Proof' completeness?

Approach: Define the requirement that a honest prover can always convince a honest verifier.

Completeness means that if the statement is true, an honest prover will be able to convince an honest verifier of this fact with a probability of 1.

Follow-up: What is 'Soundness' in this context?

Key concepts: honest actors, probability 1, proof.