A monopoly is the condition where there is only one seller of a product. People may describe a business as having a monopoly in the short term but in the long term a monopoly is not possible without the use or the threat of violence and loss of property brought upon those who would compete with the so called monopoly. So a “Natural Monopoly” is simply a business who has impressed upon government in such a way as to have government grant it an exclusive permit to sell a product in an area without competition. Normally these businesses simply use the fact that high entrance costs prohibit other firms from competing along with some graft paid to the politicians.
I think EmperorNero mentioned this above, but using the definition of “only one provider of a unique product,” technically everything is a monopoly. If I want to buy a Pepsi at a Speedway, that Speedway has the monopoly rate at that particular location. But it is a matter of ease of entry; if you could easily go across the street to a supermarket and get a Pepsi versus if the Speedway is the only provider of a Pepsi bottle, it would cease to be a monopoly in a meaningful sense.