A macro lens is one whose aberrations are corrected for photographing small objects near the lens rather than distant objects. This does not imply a different lens design; it's a subtle matter of adjusting the curvatures and separations of elements.
Since the 1970s, many of the best macro lenses have had floating elements, elements whose separation changes as you focus. As a result, they are very sharp at infinity as well as close up. I have had excellent results photographing star fields with Sigma 90-mm and 105-mm macro lenses.
At the other end of the scale, cheap zoom lenses often claim to be "macro" if they will focus on anything less than an arm's length away, even if they don't perform especially well at any distance or focal length.
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