The Collaborative International Dictionary of English v.0.48:

Lao-tse \Lao-tse\, Laozi \Laozi\prop. n.
   A Chinese philosopher who founded Taoism in the 6th-century
   b.c.

   Syn: Lao-Tzu, Lao-Tse.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Lao-zi
    n 1: Chinese philosopher regarded as the founder of Taoism (6th
         century BC) [syn: Lao-tzu, Lao-tse, Lao-zi]