Harry Thurston Peck, Harpers Dictionary of Classical Antiquities (1898)

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Chalcidĭcum

An annex or addition to a basilica (q.v.), of a nature made clear by the socalled fullonica at Pompeii which bears this name in an inscription upon its front. This shows that the chalcidicum was an entrance-hall to a public building, designed for the shelter of persons waiting to be admitted, or who might transact their business under it; it was wholly or partially roofed, and might take the form either of a deep porch, or in some cases of a cloistered court. Such a vestibule is found in many Christian basilicas; the former type occurs in St. John Lateran and Sta. Maria Maggiore at Rome, the latter in St. Ambrogio at Milan. The foundations show that a chalcidicum of this kind once existed in front of the vast basilica of Constantine at Rome.