Accepted definitions.....

as per request.....

communism:
a political theory derived from Karl Marx, advocating class war and leading to a society in which all property is publicly owned and each person works and is paid according to their abilities and needs. See also Marxism .
One for all and all for the State.....

democracy:
a system of government by the whole population or all the eligible members of a state, typically through elected representatives
capitalism and democracy are ascendant in the third world.
• a state governed in such a way : a multiparty democracy.
• control of an organization or group by the majority of its members : the intended extension of industrial democracy.
• the practice or principles of social equality : demands for greater democracy.
Majority rules and if you’re a minority, tough shit!

republic:
a state in which supreme power is held by the people and their elected representatives, and which has an elected or nominated president rather than a monarch.
Majority nominally rules but there’s always another vote!

Though a constitutional republic is not a pure democracy it necessarily has some democratic elements, such as ability of the people to elect a president (in the U.S. the majority of the population is checked here too, as popular vote of the people does not necessarily decide the winner). Nations where the head of state is not elected, as in a monarchy, as not elected but has a parliament with elected representatives that govern according to constitutional law protecting individual rights are called constitutional democratic monarchy). Both are considered liberal democracies because they protect individual liberty from majority and minority forces, while retaining some democratic elements.