Autos sacramentales (Spanish auto, “act” or “ordinance”; sacramental, “sacramental, pertaining to a sacrament”) are a form of dramatic literature which is peculiar to Spain, though in some respects similar in character to the old Morality plays of England.
The auto sacramental may be defined as a dramatic representation of the mystery of the Eucharist. At least this is the definition that would apply to the auto of the time of Calderón. It does not so well fit, however, those of the preceding century, many of which were sacramental in character only because they were presented during the feast of Corpus Christi. They are usually allegorical, the characters representing, for example, Faith, Hope, Air, Sin, Death, etc. There were some indeed, in which not a single human character appeared, but personifications of the Virtues, the Vices, the Elements, etc.
Answers.com Definition of Auto Sacramental.
New Advent Catholic Encyclopedia Comprehensive definition of this Spanish genre.
The Free Dictionary Brief definition of Auto Sacramental.
Theatre History.com Useful biography of Spanish playwright Calderon.
Theatre Database Excellent article on both Lope de Vega and Calderon.