The Collaborative International Dictionary of English v.0.48:

Placitum \Plac"i*tum\, n.; pl. Placita. [LL. See Placit.]
   1. A public court or assembly in the Middle Ages, over which
      the sovereign president when a consultation was held upon
      affairs of state. --Brande & C.
      [1913 Webster]

   2. (Old Eng. Law) A court, or cause in court.
      [1913 Webster]

   3. (Law) A plea; a pleading; a judicial proceeding; a suit.
      --Burrill.
      [1913 Webster]

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

PLACITUM. A plea. This word is nomen generalissimum, and refers to all the
pleas in the case. 1 Saund. 388, n. 6; Skinn. 554; S. C. earth. 834; Yelv.
65. By placitum is also understood the subdivisions in abridgments and other
works, where the point decided in a case is set down, separately, and
generally numbered. In citing, it is abbreviated as follows: Vin. Ab.
Abatement, pl. 3.
     2. Placita, is the style of the English courts at the beginning of the
record of Nisi Prius; in this sense, placita are divided into pleas of the
crown, and common pleas.
     3. The word is used by continental writers to signify jurisdictions,
judgments, or assemblies for discussing causes. It occurs frequently in the
laws of the Longobards, in which there is a title de his qui ad, placitum
venire coguntur. The word, it has been suggested, is derived from the German
platz, which signifies the same as area facta. See Const. Car. Mag. Cap. IX.
Hinemar's Epist. 227 and 197. The common formula in most of the
capitularies is "Placuit atque convenit inter Francos et corum proceres,"
and hence, says Dupin, the laws themselves are often called placita. Dupin,
Notions sur le Droit, p. 73.