The Collaborative International Dictionary of English v.0.48:

xi \xi\, adj.
   The Roman number symbolizing the value eleven. Used after a
   noun it may symbolize the ordinal number; as, Superbowl XI.

   Syn: eleven, 11.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

xi
    adj 1: being one more than ten [syn: eleven, 11, xi]
    n 1: the cardinal number that is the sum of ten and one [syn:
         eleven, 11, XI]
    2: the 14th letter of the Greek alphabet

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

Xi

   A VLSI design language.

   ["The Circuit Design Language Xi", S.I. Feldman, unpublished
   memo, Bell Labs, 1982].

   [Mentioned in Computational Aspects of VLSI, J.D. Ullman, CS
   Press 1984].

   (1995-02-03)