If something is indiscernible, it is so faint, small, or mixed in that you can hardly see, hear, or notice it. A sound behind a wall, a mark on dark cloth, a change in someone's face — if you almost cannot catch it, it is indiscernible. People also use it when two things are so alike that you cannot tell them apart: the difference was indiscernible. It is a careful, rather formal word. You will meet it in writing, in science, and in polite speech, not much in the playground. Though actually, speakers sometimes reach for it when they only mean “very hard to see,” not “impossible to see” — the line is a little soft in real use.

難以察覺的；難以分辨的