Open English Wordnet
Adjectives
(s)indefinable, undefinablenot capable of being precisely or readily described; not easily put into words“an indefinable feeling of terror”, “an abstract concept that seems indefinable”
(s)indefinable, indescribable, ineffable, unspeakable, untellable, unutterabledefying expression or description“indefinable yearnings”, “indescribable beauty”, “ineffable ecstasy”, “unspeakable happiness”, “unutterable contempt”, “a thing of untellable splendor”