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Great question @tonyf60. Another geek will probably be able to explain it better, but here is what I found when I researched the question.
“In music theory, the ‘A7’ is termed as a dominant seventh chord, constructed by adding a minor seventh to a major triad. The term ‘dominant’ here refers to the fifth degree of a diatonic scale, called the ‘dominant’. In this case, ‘A’ being the dominant of ‘D’, the ‘A7’ chord becomes the dominant seventh of the ‘D’ scale.