To say that something is "ineffable" means that it cannot or should not be expressed in spoken words (as with the concept of true love or some taboo). It is generally used to describe a feeling, concept or aspect of existence that is too great to be adequately described in words, or that inherently (due to its nature) cannot be conveyed in dualistic symbolic human language, but can only be known internally by individuals.

Quotations

"Whereof one cannot speak, thereof one must be silent." — Ludwig Wittgenstein

"The Tao that can be told is not the eternal Tao; the name that can be named is not the eternal name." — the Dao De Jing

"What can't be said, can't be said. And it can't be whistled, either." — F. P. Ramsey

"What cannot be spoken in words, but that whereby words are spoken." — Kenopanishad

"We shall grapple with the ineffable, and see if we may not eff it after all." — Douglas Adams in Dirk Gently's Holistic Detective Agency

"I'm in the business of effing the ineffable." — Alan Watts

"You can't second guess ineffability, I always say." — Aziraphale in Good Omens

Things said to be ineffable

Things said to be essentially incommunicable

• The nature of qualia (sensory experiences), such as colors or flavors
• The nature of dreams
• The nature of emotions (with love being a prominent example)
• The nature of religious experiences, e.g. Søren Kierkegaard's analysis of Abraham in Fear and Trembling, Problemata III, and in particular the mystic's realization of nonduality
• The near-death experience
• The experience of birth
• The psychedelic experience is largely considered ineffable to psychologists, philosophers and psychonauts alike
• The musical experience, following Theodor Adorno, Vladimir Jankélévitch, among others
• The human soul (see also sentience and the hard problem of consciousness)
• The name of a god or gods, in some religions¹
• The Dao
• The ultimate definitions of good and evil

Things said to be incommunicable due to incomprehensibility

• The pre-big bang universe
• The size of the universe
• Pre-birth
• Post-death
• The concept of Infinity
• A square with 3 sides or any other illogical proposition

Phrases considered too great to be spoken

• The Tetragrammaton (YHWH, by orthodox Jewish tradition)
• The "Will of Bob" in Mostly Harmless, part of The Hitchhiker's Guide to the Galaxy

References