indistinguishable
UK: ˌɪndɪˈstɪŋɡwɪʃəbl | US: ˌɪndɪˈstɪŋɡwɪʃəbl
adj. impossible to differentiate or tell apart
adj. lacking clearly distinguishable features
The word combines the negative prefix "in-" (from Latin, meaning "not") with "distinguish" (from Latin "distinguere," meaning "to separate or mark as different") and the suffix "-able" (indicating capability). Originally, "distinguish" referred to physically marking differences, but over time, it broadened to include perceptual or conceptual differentiation. "Indistinguishable" thus logically evolved to describe things that cannot be separated or differentiated, whether physically or abstractly.
The twins were nearly indistinguishable in appearance.
The counterfeit bills were so well-made they were indistinguishable from real ones.
At dusk, the colors of the sky became indistinguishable.
Their voices on the phone were indistinguishable to most listeners.
The two theories are virtually indistinguishable in their practical implications.