WordNet (r) 3.0 (2006):

matter of law
    n 1: a disputed legal contention that is generally left for a
         judge to decide [syn: question of law, matter of law]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

MATTER OF LAW, pleading. That which goes in avoidance of a declaration or
other pleading, on the ground that the law does not authorize them. It does
not deny the matter or fact contained in such pleading, but admitting them
avoids them. Bac. Ab. Pleas, &c. G 3. Matter of law, is that which is
referred to the decision of the court; matter of fact that which is
submitted to the jury.