Full Idea
When the conditions for a rational being having knowledge of the nature of a world are offered as the grounds for the judgement that such a world must have certain characteristics, we have transcendental necessity.
Gist of Idea
Transcendental necessity is conditions of a world required for a rational being to know its nature
Source
Harré,R./Madden,E.H. (Causal Powers [1975], 1.V.B)
Book Reference
Harré,R/Madden,E.H.: 'Causal Powers: A Theory of Natural Necessity' [Blackwell 1975], p.20
A Reaction
It seems like a rather hard call to decide whether such characteristics pertain to the world, or to the mind of the rational being. Kant is obviously behind this one. You must read his first Critique at least four times to evaluate it.