Synonyms of the word

imperceptible

• adj. Not perceptible, not detectable, too small in magnitude to be observed.

impalpable

• adj. Not able to be perceived by the senses (especially by touch); intangible or insubstantial.

inaudible

• adj. Unable to be heard or not loud enough to be heard.

incognizable

• adj. Not cognizable; incapable of being recognised.

incognoscible

• adj. (rare, dated) Not capable of being known; incomprehensible.

indiscernible

• adj. Not capable of being discerned, of being perceived.
• adj. Not capable of being distinguished from something else.
• n. (chiefly philosophy) Something which is incapable of being discerned.

insensible

• adj. Unable to be perceived by the senses.
• adj. Incapable or deprived of physical sensation.
• adj. Unable to be understood; unintelligible.
• adj. Not sensible or reasonable; meaningless.
• adj. Incapable of mental feeling; indifferent.
• adj. Incapable of emotional feeling; callous; apathetic.

invisible

• adj. Unable to be seen; out of sight; not visible.
• adj. Not appearing on the surface.
• adj. (Internet) Apparently, but not actually, offline.
• adj. (psychology) That is ignored by a person.
• v. To invisiblize.
• n. (obsolete) An invisible person or thing; specifically, God, the Supreme Being.
• n. (obsolete) A Rosicrucian; so called because avoiding declaration of his craft.
• n. (obsolete) One of those (as in the 16th century) who denied the visibility of the church.

subliminal

• adj. (of a stimulus) Below the threshold of conscious perception, especially if still able to produce a response.

undetectable

• adj. Unable to be detected; not detectable.

unhearable

• adj. That cannot be heard.

unobservable

• adj. That cannot be observed.
• n. Something that cannot be observed.

unperceivable

• adj. Not perceivable; imperceptible.

unseeable

• adj. invisible.
• adj. unimaginable.