From Longman Business Dictionary
oligopolyol·i·gop·o·ly /ˌɒlɪˈgɒpəliˌɑːlɪˈgɑː-, ˌoʊ-/ noun (plural oligopolies) [countableC, uncountableU]ECONOMICS a situation in which a very small number of companies produce a particular thing or provide a particular service, or the group of companies itselfCompetition authorities would not approve of the creation of a defence oligopoly.
→ compare monopoly