The definition is when a market is dominated by a select few companies.
The typical examples used are:
Supermarkets (ASDA, Tesco, Sainsburys and Morrisons)
Video gaming consoles (Nintendo, Microsoft, Sony)
Soft drinks (Mars, Pepsi and Coca-Cola)
Fast food (McDonald's, Burger King, KFC)
Supermarkets, for example - There is Netto, Aldi, Lidl and so on, but the market is still dominated, as is shown by how much of the market share the big four have.
Likewise, in video gaming consoles - There was the NGear and Dreamcast, but they don't mean it isn't an oligopoly.