Imperceptible means impossible or nearly impossible to notice or detect. When something is imperceptible, it's so slight, so gradual, or so subtle that your senses can't pick it up.

Think about how a tree grows: each day the change is imperceptible. You can't actually see it getting taller if you watch it, even for hours. But come back after summer vacation and you'll notice it shot up several inches. The growth was happening all along, just at an imperceptible rate.

The same applies to other gradual changes. When you're reading a book and daylight fades, the change in brightness is so imperceptible that you might not realize how dark it's gotten until you can barely see the page. The temperature in a room might rise at an imperceptible rate during a hot afternoon.

Scientists use sensitive instruments to measure imperceptible things that humans can't detect on their own, like tiny earthquakes, faint radio waves from space, or chemical traces in the air. What's imperceptible to us might be quite noticeable to other creatures: dogs hear high-frequency sounds imperceptible to human ears, and hawks spot movements imperceptible to us from hundreds of feet in the air.

The opposite would be perceptible, meaning noticeable or detectable.