Tangent (geometry)
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A tangent line is a linear approximate to a curve. That is, if you zoom in very closely, the tangent line and the curve will become indistinguisable from each other.
Locally, a tangent line intersects a curve in a single point. However, if a curve is neither convex nor concave, it is possible for a tangent line to intersect a curve in additional points. For instance, the tangent line of the curve 
at 
intersects it in 1 point, while the tangent line at 
intersects it in 2 points and the tangent line at 
intersects it in infinitely many points (and is in fact the tangent line at each point of intersection).
At a given point, a curve may have either 0 or 1 tangent lines. For the graph of a function, the condition "having a tangent line at a point" is equivalent to "being a differentiable function at that point." It is a fairly strong condition on a function -- only continuous functions may have tangent lines, and there are many continuous functions which fail to have tangent lines either at some points (for instance, the absolute value function 
at 
) or even at all points!
