默契is often translated as "tacit understanding" in English, but this translation does not fully reflect the meaning of the Chinese word. In other words, there is no single-word vocabulary in English that is an exact equivalent of 默契。 To translate something like 他们夫妻俩结婚多年，很有默契，不用说什么，使个眼色对方就了解另一方的意思, you will have to describe the situation, such as: Those two have been married for so long that neither has to say anything for them to understand each other, as if they can read each other's minds. Or Those two have been married for so long that one can read the other's thoughts without any exchange of words.