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A straight line that a curve approaches more and more closely but never touches as the curve goes off to infinity in one direction.
For example, the x and y axes are asymptotes to the curve of xy = 1. As x approaches infinity, the curve is getting closer and closer to the x axis but can never meet the x axis. Similarly, as y approaches infinity, the curve is also getting closer and closer to the y axis but can never reach the y axis.
