A vowel system where (a) there is no open vowel and/or (b) for mid and high regions there is at least one front vowel which is not matched by a back vowel, or vice versa. Below are three examples of defective systems:

Campa (Peru)
i           –
e         o
     a

Pirahã (Brazil)
i           –
–         o
     a

Arapaho (Wyoming, Oklahoma)
ɪ iː           ʊ uː
ɛ ɛː          ɔ ɔː
          –

For further information see Ferrar Disner, S. (1984) Insights on vowel spacing. In Maddieson I. (1984) Patterns of sounds. Cambridge University Press, pp.136-155.