The Collaborative International Dictionary of English v.0.48:

Sederunt \Se*de"runt\, n. [L., they sat, fr. sedere to sit.]
   A sitting, as of a court or other body.
   [1913 Webster]

         'T is pity we have not Burns's own account of that long
         sederunt.                                --Prof.
                                                  Wilson.
   [1913 Webster]

   Acts of sederunt (Scots Law), ordinances of the Court of
      Session for the ordering of processes and expediting of
      justice. --Bell.
      [1913 Webster]

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

ACTS OF SEDERUNT. In the laws of Scotland, are ordinances for regulating the
forms of proceeding, before the court of session, in the administration of
justice, made by the judges, who have a delegated power from the legislature
for that purpose. Ersk. Pr. L. Scot. B. 1, t. 1, s. 14.