The Collaborative International Dictionary of English v.0.48:

Minibus \Min"i*bus\, n. [L. minor less + -bus, as in omnibus.]
   1. A kind of light passenger vehicle, carrying four persons.
      [archaic]
      [1913 Webster]

   2. A type of bus (omnibus) similar in general appearnace to a
      standard-sized bus, but smaller and having a smaller
      passenger capacity, typically about 15 persons. It is
      usually used for transportation over relatively short
      distances.
      [PJC]

WordNet (r) 3.0 (2006):

minibus
    n 1: a light bus (4 to 10 passengers)