Monopoly

 If a Competitive Firm is a price taker, so a monopoly firm is a price maker.
Terms :

• There is one firm in the market
• There is not other seller which can sell a substitution for output. 
• There is a barriers to other firm to entry.

Why monopolist arise?  Main cause monopoly is "barriers to entry"
Monopoly Resources
Although exclusive ownership of a key resource is a potential source of monopoly, in practice monopolies rarely arise for this reason.
A monopolist revenue
TR = P * Q = TR
TR/Q = AR = Q

Marginal Revenue
delta TR / delta Q = MR