aromatic shrubby plant, early 15c., from Old French basile (15c., Modern French basilic), from Medieval Latin basilicum, from Greek basilikon (phyton) "royal (plant)," from basileus "king" (see Basil). It was so called, probably, because it was believed to have been used in making royal perfumes. In Latin, the word was confused with basiliscus (see basilisk) because it was supposed to be an antidote to the basilisk's venom.
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 this was the portico of the archon basileus, the official who dispensed justice there. The word is thus from the 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.