Picture a hospital, a school and a bus route, and then ask the plain question: who pays, and who decides? Social democracy is one answer to that pair of questions. Taxes pay, so that everyone can use the things, and elections decide, so nobody takes that power for granted. Hold both halves at once and the word opens up. Said of a set of ideas it is that answer itself; said of a place it is a country that has given that answer, which is why the grammar turns countable the moment you point at one.