What are the barriers to entry that shield pure monopolies from competition?

A barrier to entry is a term for something that stops competing firms from entering a market. Monopolies are protected by significant barriers to entry, otherwise, they would no longer hold a monopoly. Common examples are:

• Government regulation or public ownership. In some markets, governments decide to own the firm outright, as is the case with many municipal water services. Sometimes they charter a single firm to hold the monopoly and set its price. Other times, regulations make it impossible for new firms to enter the market.
• Trade secrets. If a firm is the only firm that knows how to produce the product, the secret method will act as a barrier to entry.
• Patents and copyrights. Some governments create temporary monopolies over inventions or artistic products. Governments do this to promote innovation and creation in their countries.
• Private property. Sometimes a firm will own the property and can keep out any competition. This occurs at many sports stadiums and theme parks. A firm might also hold a monopoly over the materials used to make a specific product, which creates a monopoly.