Weakly defective varieties

A projective variety is `-weakly defective' when its intersection with a general -tangent hyperplane has no isolated singularities at the points of tangency. If is -defective, i.e. if the -secant variety of has dimension smaller than expected, then is also -weakly defective. The converse does not hold in general. A classification of weakly defective varieties seems to be a basic step in the study of defective varieties of higher dimension. We start this classification here, describing all weakly defective irreducible surfaces. Our method also provides a new proof of the classical Terracini's classification of -defective surfaces.