unobservable (adjective) unobservable: imperceptible
Sinonime: ephemeral,
evanescent,
fine,
gradual, impalpable, imponderable, inappreciable, inaudible, inconsiderable,
inconspicuous, indiscernible,
indistinct, indistinguishable,
infinitesimal, insensible,
insignificant,
intangible,
invisible, microscopic,
minute, momentary, shadowy,
slight,
small,
subtle,
tiny,
trivial, undetectable, unnoticeable,
vague