1. 难以确切表达的;难以确定的;模糊不清的
    An indefinable quality or feeling cannot easily be described.

1. not capable of being precisely or readily described
    not easily put into words

    e.g. an indefinable feeling of terror
           an abstract concept that seems indefinable

    Synonym: undefinable

2. defying expression or description

    e.g. indefinable yearnings
           indescribable beauty
           ineffable ecstasy
           inexpressible anguish
           unspeakable happiness
           unutterable contempt
           a thing of untellable splendor

    Synonym: indescribableineffableunspeakableuntellableunutterable