  The number of apparent double points of a smooth, irreducible projective
variety $X$ of dimension $n$ in $\Proj^{2n+1}$ is the number of secant lines to
$X$ passing through the general point of $\Proj^{2n+1}$. This classical notion
dates back to Severi. In the present paper we classify smooth varieties of
dimension at most three having one apparent double point. The techniques
developed for this purpose allow to treat a wider class of projective
varieties.

Algebraic Geometry