To say that something is "ineffable" means that it cannot or should not be spoken.

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

"If a person can't communicate, the very least he can do is to shut up." - Tom Lehrer

Things that are said to be ineffable include:

• The Tetragrammaton, or Yahweh (by orthodox Jewish tradition)
• Names of various villains in works of fiction, e.g. Lord Voldemort in the Harry Potter books or the "Dark One" in The Wheel of Time sequence.
• The nature of qualia (sensory experiences)
• The nature of spiritual experiences