ETYM Pref. im- not + perceptible: cf. French imperceptible.
Impossible or difficult to perceive by the mind or senses; SYN. unperceivable.

ETYM Pref. in- not + appreciable: cf. French inappréciable.
Too small to make a significant difference.

ETYM Pref. in- not + discernible: cf. French indiscernable.
1. Barely able to be perceived; SYN. insensible, undetectable.
2. Difficult or impossible to perceive or discern.

Not capable of being distinguished or differentiated; SYN. undistinguishable.

Imperceptibly; intangibly

In an imperceptible manner or to an imperceptible degree; SYN. unnoticeably, observably.

Imperceptibly; insignificantly, in an unimportant manner, trivially

Indiscriminately; so as to be impossible to differentiate

In an undiscernible manner, imperceptibly, in a manner than cannot be sensed