Things Considered to be Ineffable
A few things said to be essentially incommunicable, according to http://www.artandpopularculture.com/Ineffability
The innate properties of any form of perception, at any level (such as awareness, self-consciousness, or other perceptions of the perceived to an arbitrary extent)
The nature of qualia (sensory experiences), such as colors or flavors (Although this has been challenged recently by research on ideasthesia finding that qualia are connected in semantic-like networks.)
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 experience of death
Aesthetic content and experiences
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 meaning of life
Things said to be incomprehensibly incommunicable
Pre-mortal-existence
Post-death
Higher dimensions
Things whose expression are regarded as sacred, or otherwise socially prohibited
The name of a god or gods, in some religions
The Tetragrammaton (יהוה, by orthodox Jewish tradition).