Sola scriptura (Sōlā scrīptūrā Latin ablative, "by scripture alone") is the assertion that the Bible as God's written word is self-authenticating, clear (perspicuous) to the rational reader, its own interpreter ("Scripture interprets Scripture"), and sufficient of itself to be the final authority of Christian doctrine.
Thanks to good ol' heretic Martin Luther (who used Sola scriptura as one of the main doctrines of his Reformation), we now have this aggravating, exasperating, bang-my-head-on-the-wall-maddening continuous unending stream of anti-Christian rhetoric that basically tries to block any reasonable Christian argument on controversial moral issues using a line of reasoning that's summed up as follows:
"The Old Testament of the Bible condones child slavery and commands death to the disobedient, to name a few disturbing examples of Biblical immorality. Therefore, Christianity has no right to claim authority over what is immoral and what isn't."
(One day I heard a group of non-Christians arguing this, and one of them is a Jew. Pathetic.)
(Another aside: There is a similar line of reasoning involving the murders committed by high-ranking Catholics in the past, yet since those actions were not supported by any Catholic dogma, nor by the Catechism, this isn't really analogous to the issue above.)
Oh how I wish Luther could some day come back from the dead and answer this for his followers. Would be fun to watch. Meanwhile, I'm trying to figure out how to remind myself that, after all, the attack is technically upon the Protestants, not us Catholics, and that I shouldn't fuss about it too much. Yet it's difficult when so many non-Christians (and, surprise surprise, ill-educated Christians) fail to recognize the differences between the actual Christian Church and the very popular heresy called Protestantism. So now even Catholics can't argue with immoral people without the usual "Well just look at what Leviticus says about who should be stoned to death! *smirk*"
Just to be clear, Christian morality is based upon... wait for it... the teachings of Christ as passed down to us by his Apostles, not upon Jewish Law. The Church recognizes that a lot of Jewish scripture can somehow be interpreted to reveal Christian truths, which is why we have the Old Testament. (Besides, Jewish scripture is a great way of providing Christians with a description of the culture in which Christ lived, as he himself was a Jew.) Yet though we have the Torah, etc. as part of our own Scriptures, the laws given by Moses to the people of Israel do not necessarily apply to us, especially the majority of Christians who are gentile!
Now you know that when you catch me face-palming after hearing the stale "immoral Bible" argument, at that point I'm actually thinking "Luther, you idiot, look at what you've done!"
(Note: I call Luther "idiot", though of course he wasn't. I do this partly because he himself enjoyed calling people names, and partly because Sola Scriptura is idiotic. None of these reasons justify calling him an idiot, but this is a blog rant, dammit...)