indefinable

From in- +‎ definable.

indefinable (not comparable)

1. That which cannot be precisely defined or put into words.
   • 1936, Rollo Ahmed, The Black Art, London: Long, page 178:

     They apparently have a theory that the indefinable psychic gift that they call magic, and which we might designate "psychicism", can be acquired by accosting the soul of a murdered man in a burial ground at midnight.

2. That which defies description; indescribable.

• undefinable (less common)

• definable

• indefinability

indefinable (plural indefinables)

1. Anything that cannot be defined.