diad

Who can tell me what the meaning of "diad" is? Thank you! Interestingly, this seems to be a spelling mistake! The word 'diad' does not appear in the dictionary but 'dyad' does. It means two entities treated as one and is related to 'triad' which appears later in the paragraph. (Tilly)

Thanks for your answer, Tilly! You are always helping me.(Shuang)