The Collaborative International Dictionary of English v.0.48:

Pibroch \Pi"broch\, n. [Gael. piobaireachd pipe music, fr.
   piobair a piper, fr. pioba pipe, bagpipe, from English. See
   Pipe, n.]
   A Highland air, suited to the particular passion which the
   musician would either excite or assuage; generally applied to
   those airs that are played on the bagpipe before the
   Highlanders when they go out to battle. --Jamieson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pibroch
    n 1: martial music with variations; to be played by bagpipes