Not palpable; that cannot be felt; extremely fine, so that no grit can be perceived by touch.

Not material; intangible; incorporeal.

incapable of being perceived by the senses especially the sense of touch; "the intangible constituent of energy"- James Jeans

Not apprehensible, or readily apprehensible, by the mind; unreal; as, impalpable distinctions.

imperceptible to the senses or the mind; "an impalpable cloud"; "impalpable shadows"; "impalpable distinctions"; "as impalpable as a dream"

"Describing a structure in the body that cannot be detected by feeling with the hand." -- Clinical Standards for Breast Screening.