Two curves both containing the point 
are tangent at 
if they share the same tangent
line at 
.
Tangent Curves
See also
Osculating Curves, Tacnode, Tangent Circles, Tangent LineThis entry contributed by David Terr
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Cite this as:
Weisstein, Eric W., with contributions by David Terr. "Tangent Curves." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/TangentCurves.html