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Something is dappled when it has patches of a darker or lighter shade of color. Some horses are dapple grey.
It is dark when there is not much light and you have trouble seeing anything. It gets dark at night time, or when you close the curtains and shut the light out. The opposite of dark is light or bright.
Dark may also be used to describe something that has much deeper colors than something else, or is close to black. The opposite of this kind of dark is pale.
Someone is deaf if they cannot hear things properly, or they cannot hear things at all. People who are only partially deaf can use a hearing aid to help them hear things properly again.