Final answer:
The word 'opaque' indicates that the window in the sentence does not allow light to pass through, making it completely dark as opposed to the other windows that were illuminated by candlelight.
Step-by-step explanation:
The word opaque in the sentence "While the other windows shone with candlelight, the opaque window was completely dark." best defines the window as not transparent. This means that light does not pass through the opaque window, hence no visibility from the other side, unlike transparent or even translucent materials where light can pass through to various degrees. In contrast to the other windows that were illuminated by candlelight, the opaque window remained dark, indicating that it was impossible for light to penetrate and be seen through it.