The Collaborative International Dictionary of English v.0.48:

Practise \Prac"tise\, v. t. & i.
   See Practice.
   [1913 Webster]

   Note: The analogy of the English language requires that the
         noun and verb which are pronounced alike should agree
         in spelling. Thus we have notice (n. & v.), noticed,
         noticing, noticer; poultice (n. & v.); apprentice (n. &
         v.); office (n. & v.), officer (n.); lattice (n.),
         latticed (a.); benefice (n.), beneficed (a.), etc. Cf.
         sacrifice (?; n. & v.), surmise (?; n. & v.), promise
         (?; n. & v.); compromise (?; n. & v.), etc. Contrast
         advice (?; n.), and advise (?); device (?), and devise
         (?), etc.
         [1913 Webster]

WordNet (r) 3.0 (2006):

practise
    v 1: engage in a rehearsal (of) [syn: rehearse, practise,
         practice]
    2: carry out or practice; as of jobs and professions; "practice
       law" [syn: practice, practise, exercise, do]
    3: learn by repetition; "We drilled French verbs every day";
       "Pianists practice scales" [syn: drill, exercise,
       practice, practise]