Should “data” be singular or plural?

For more than a millennium after the fall of Rome, educated Europeans were distinguished by their knowledge of Latin. One of the three subjects of the trivium—the basic tier of a classical education, itself based on a Roman model—was Latin grammar. Europeans have long since stopped writing primarily in Latin, but learned people are still expected to be able to deduce that to “decimate” means to destroy a tenth of something (a mutinous legion was punished in this way), or sprinkle annus mirabilis and mutatis mutandis into their speech.