imperceptibly

UK: /ˌɪmpə^rˈsɛptɪbli/

WordReference Random House Learner's Dictionary of American English © 2025

im•per•cep•ti•ble /ˌɪmpɚˈsɛptəbəl/   adj. 

1. very difficult to notice; subtle;
   not able to be easily perceived:
   At first the uphill slope of the road was imperceptible, but it became steeper.

im•per•cep•ti•bil•i•ty /ˌɪmpɚˌsɛptəˈbɪlɪti/  n. [uncountable]
im•per•cep•ti•bly, adv. See -cep-.

WordReference Random House Unabridged Dictionary of American English © 2025

im•per•cep•ti•ble  (im′pər sep′tə bəl), adj. 

1. very slight, gradual, or subtle:
   the imperceptible slope of the road.
2. not perceptible;
   not perceived by or affecting the senses.

n. 

3. something not capable of being perceived by the senses:
   metaphysical speculation about imperceptibles.

• Medieval Latin imperceptibilis. See im-², perceptible
• 1520–30

im′per•cep′ti•bil′i•ty, im′per•cep′ti•ble•ness, n. 
im′per•cep′ti•bly, adv. 
Synonyms

• 2. hidden, unperceivable, undetectable.