隙 - crevice, fissure, discord

gap; opening; aperture; crevice; crack; chink; space

gap; opening; room; space

peeping

chink in the armor (armour)

synaptic cleft

gap; opening; space

vacant space; aperture; gap; opening

void ratio

one must constantly be on one's guard with ...; one cannot be too careful of ...

porosity

without any weak spot; watertight (e.g. reasoning); impeccable

time flies; like watching a white horse run by through a gap in the wall (Zhuangzi)