Which word denotes an undeniable truth?

The word that denotes an undeniable truth is "Fact." A fact is something that is objectively verifiable and can be demonstrated through evidence or observation. It exists independently of opinions or interpretations, making it a reliable foundation for knowledge and understanding. In contrast, an illusion denotes a deceptive appearance or perception that does not correspond to reality, which means it can mislead rather than reveal a truth. Fiction refers to narrative content that is created from imagination rather than based on reality or facts, indicating that it is not inherently true. A paradox represents a seemingly self-contradictory statement that may reveal a deeper truth upon closer examination, but it does not serve as a straightforward declaration of fact. Therefore, among the choices provided, "Fact" is the only term that directly signifies an undeniable truth.