Monopoly
Plain definition
A market with a single seller of a product with no close substitutes, protected by barriers to entry.
In economics
A monopolist faces the entire downward-sloping market demand curve, sets output where MR = MC, and charges the highest price the demand curve allows — producing less and charging more than a competitive industry would. Without competitive pressure, monopolies earn long-run economic profit, which is why governments regulate or break them up.
Example
A patented drug with no generic equivalent is a temporary monopoly: one firm controls supply and sets the price.
Monopoly is confused with 'a very large company.' Size alone does not make a monopoly — it is being the only seller that matters, and barriers to entry (patents, control of resources, network effects) are what sustain it. A big firm in a competitive market is not a monopolist.