The Collaborative International Dictionary of English v.0.48:

initialise \initialise\ v. t.
   initialize. [Chiefly Brit.]
   [PJC]

WordNet (r) 3.0 (2006):

initialise
    v 1: assign an initial value to a computer program [syn:
         initialize, initialise]
    2: divide (a disk) into marked sectors so that it may store
       data; "Please format this disk before entering data!" [syn:
       format, initialize, initialise]

The Free On-line Dictionary of Computing (30 December 2018):

initialise

    To give a variable its first value.  This may
   be done automatically by some languages or it may require
   explicit code by the programmer.  Some languages allow
   initialisation to be combined with variable definition,
   e.g. in C:

   	int i = 0;

   Failing to initialise a variable before using it is a common
   programming error, but one which compilers and automatic
   checkers like lint can easily detect.

   (1997-06-08)