Merriam-Webster defines "veridical" as truthful, veracious and non illusory. It stems from the Latin "veridicus", composed of Latin verus, meaning "true", and dicere, which means "to say". For example, the statement "Paul saw a snake" asserts the truthfulness of the claim, while "Paul did see a snake" is an even stronger assertion.
The formal definition of veridicality views the context as a propositional operator (Giannakidou 1998).
A propositional operator F is veridical iff Fp entails p, that is, Fp → p; otherwise F is nonveridical.
Additionally, a nonveridical operator F is antiveridical iff Fp entails not p, that is, Fp → ¬p.
For temporal and aspectual operators, the definition of veridicality is somewhat more complex:
For operators relative to instants of time: Let F be a temporal or aspectual operator, and t an instant of time.
F is veridical iff for Fp to be true at time t, p must be true at a (contextually relevant) time t′ ≤ t; otherwise F is nonveridical.
A nonveridical operator F is antiveridical iff for Fp to be true at time t, ¬p must be true at a (contextually relevant) time t′ ≤ t.
For operators relative to intervals of time: Let F be a temporal or aspectual operator, and t an interval of time.
F is veridical iff for Fp to be true of t, p must be true of all (contextually relevant) t′ ⊆ t; otherwise F is nonveridical.
A nonveridical operator F is antiveridical iff for Fp to be true of t, ¬p must be true of all (contextually relevant) t′ ⊆ t.