One reason we need government is that the invisible hand can work its magic only if the government enforces the rules and maintains the institutions that are key to a market economy. Most important, market economies need institutions to enforce property rights so individuals can own and control scarce resources.
market failure
a situation in which a market left on its own fails to allocate resources efficiently
externality
the impact of one person’s actions on the well-being of a bystander
market power
the ability of a single economic actor (or small group of actors) to have a substantial influence on market prices