Ineffable

ineffable [ĭn-EF-ə-bəl]
adj. incapable of being expressed or described in words; inexpressible

[From Latin ineffābilis, from in- ("not") + effābilis ("utterable") from ex- ("out") + fārī ("to speak")]

The primary function of ineffable is as an intensifier. "It wasn't just that the girl was beautiful. Her beauty was beyond description. It was ineffable." Things which defy description or expression are ineffable: "ineffable ecstasy"; "inexpressible anguish"; "unspeakable happiness"; "unutterable contempt"; and "a feast of untellable splendor."

A second way ineffable is used is to describe a sort of gray area. Christopher Hitchens, here in Slate, for example, writes that Barack Obama has reached an undefinable place, a destination which defies description because the boundaries of that place are opaque:

President Barack Obama's visit to Europe afforded us an opportunity to gauge the strengths and weaknesses of his style in operation. And, even though he has almost attained the Holy Grail of public relations—in other words, he is practically at that ineffable and serene point where he gets good press for getting good press—there may come a time when even his trans-Atlantic admirers will have to take a second look.

A third way in which ineffable is used is with sacred concepts that are "not to be uttered" or "taboo." In Judaism, traditionally, it is taboo to speak or write the name of God. God is ineffable to the Jews. The idea is that by putting a description on something sacred, you are inevitably limiting that thing and bringing it down to your level. In his book, A History of Heaven, Jeffrey Burton Russell described the sacred as incomprehensible, and therefore indefinable:

Heaven itself is ineffable, beyond words. The term ineffabilis was established in theology in the fifth century by Augustine (354-430), who said that it is easier to say what God is not than to say what he is. God is not only incomprehensible to humans but is himself beyond all categories; heaven is therefore also beyond categories. Yet we have no way of discussing heaven except in the only speech we know, human language.