"defense, justification," 1784, the Latin form of apology (q.v.); popularized by J.H. Newman's "Apologia pro Vita Sua" (1864). It preserves the older sense of the English apology and the sense of the Greek original, especially as used by the Church fathers.
In common Greek, apologia refers to the speech that an accused person delivered in court, rejecting the charges filed against him or her. The apologists of the second century chose this term because they wanted to show that the charges filed against Christians were unjustified and that the truths of their faith could be described and defended. An apologia was dedicated to the Roman empoeror, who certainly never read it. [Max L. Stackhouse, "Apologia," 1988]
In classical Greek, "a well-reasoned reply; a 'thought-out response' to the accusations made," as that of Socrates. The original English sense of "self-justification" yielded a meaning "frank expression of regret for wrong done," first recorded 1590s, but this was not the main sense until 18c. Johnson's dictionary defines it as "Defence; excuse," and adds, "Apology generally signifies rather excuse than vindication, and tends rather to extenuate the fault, than prove innocence," which might indicate the path of the sense shift. The old sense has tended to shift to the Latin form apologia (1784), known from early Christian writings in defense of the faith.