Answer: PROOF

Proof refers to evidence or a demonstration that shows something to be true or valid. In the context of the clue, "conjecture's lack," it suggests that a proof is the absence of mere speculation or guesswork. In other words, a proof is a supported and validated statement or argument, as opposed to an unconfirmed or uncertain conjecture.

Note: The provided meaning of the answer is for the latest New York Times crossword puzzle clue: Mathematician's creation. Explanations are generated by AI and may be inaccurate.