1540s, "type of building based on the Athenian royal portico, large oblong building with double columns and a semicircular porch at the end," from Latin basilica "building of a court of justice," from Greek (stoa) basilike "royal (portal)," in Athens the portico of the archon basileus, the official who dispensed justice in Athens; from fem. adjective of basileus "king" (see Basil).
In Rome, the style of building used for halls of justice, many of which were subsequently appropriated as churches, and so it became a standard plan for new churches. The word is applied to the seven principal Roman churches founded by Constantine. The specific reference to Christian churches in English is attested by 1560s.