For example, if and , a singularity means the vanishing of the gradient, or equivalently, the linear terms of the Taylor's polynomial of some degree greater than 1. If one has a singularity, one may look at the second-order terms in the Taylor's polynomial.
1989, J. Berry, John Stephen Berry, A. Norcliffe, S. Humble, Introductory Mathematics Through Science Applications, Cambridge University Press, page 211: