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:
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.