The Collaborative International Dictionary of English v.0.48:

Lese majesty \Lese` maj"es*ty\, Leze majesty \Leze`
maj"es*ty\(l[=e]z` m[a^]j"[e^]s*t[y^]) n. [F. lese-majest['e],
   fr. L. laesus, fem. laesa, injured (see Lesion) + majestas
   majesty; that is, crimen laesae majestatis.] [Written also
   l[`e]se majesty.]
   1. (Law) Any crime committed against the sovereign power, or
      against a ruler.
      [1913 Webster]

   2. Any affront to the dignity of an eminent or respected
      person.
      [PJC]

   3. An attack against an institution or custom revered by the
      majority in a society.
      [PJC]

WordNet (r) 3.0 (2006):

lese majesty
    n 1: a crime that undermines the offender's government [syn:
         treason, high treason, lese majesty]