— impalpable
— impartial
— imperative
— imperceptibly
— imperceptive
— imperfect
— imperfection
— imperial
— imperil
— imperiled
— imperishable
— impermeable
— imperturbable
— impracticable
— imprecise
— impregnable
— impressible
— impressive
— improbable
— inaudible
— incognisable
— incognizable
— incognoscible
— indiscernible
— inferential
— infertile
— insensible
— invisible
— subliminal
— undetectable
— unhearable
— unobservable
— unseeable