Tangent snaps to the point on an arc, circle, ellipse, or planar spline that forms a tangent to another object.
When drawing a circle with the three-point method, for example, you can use the Tangent object snap to construct a circle that is tangent to three other circles.
You can create tangents with arcs and circles that are part of a block only if the block has been uniformly scaled. Also, the object's thickness direction must be parallel to the current UCS. For both splines and ellipses, the other point you specify must be on the same plane as the snap point.