imperceptible

形容词 imperceptible:

1. impossible or difficult to perceive by the mind or senses
   同义词：unperceivable

1. The difference between the two colors is imperceptible.
   这两种颜色之间的差别很难区别。
2. There is an imperceptible change in temperature.
   温度有细微的变化。

im.per.cep.ti.ble

adj.（形容词）
Impossible or difficult to perceive by the mind or senses:
无法察觉的：感觉不到的或很难觉察的：
例句：
an imperceptible drop in temperature.
难以觉察的气温下降

So subtle, slight, or gradual as to be barely perceptible:
不知不觉的：极细微的、轻的或缓慢的而几乎感觉不到的：
例句：
an imperceptible nod.
轻轻的点头

【引伸】
im.percep.tibil.ity 或
im.percep.tibleness
n.（名词）
im.percep.tibly
adv.（副词）