They were talking about gazelles on the business news earlier, which confused me at first until I realised that the gazelles in question were not antelopes, but small businesses. Gazelles are not just any old small businesses; to be a gazelle a small company has to be exceptionally fast-growing over a number of years (and in fact, it can soon stop being a small company, but can become quite large). People's definition of what constitutes an exceptional growth rate differs, but many economists talk in terms of 20% or more per year.
According to this Financial Times glossary entry, gazelles form only between about 5 and 10% of most economies. Many hi-tech companies such as Apple, Microsoft and Yahoo were gazelles in their early days, but other once fast-growing companies, which could also be considered gazelles when growth rate is taken into account, such as Facebook and Twitter, have not managed to convert this into the high turnover and profit growth usually considered necessary to be a gazelle.