# What are quasi-regular singularities?

The book EXACT SPACE-TIMESIN EINSTEIN’SGENERAL RELATIVITY by Podolsky and Griffiths has a section on Taub-Nut space-time metrics and there is defines the singularity made in the Taub metric as quasi-regular singularity $$\mathrm{d} s^{2}=-f(r)\left(\mathrm{d} t+4 l \sin ^{2} \frac{1}{2} \theta \mathrm{d} \phi\right)^{2}+\frac{\mathrm{d} r^{2}}{f(r)}+\left(r^{2}+l^{2}\right)\left(\mathrm{d} \theta^{2}+\sin ^{2} \theta \mathrm{d} \phi^{2}\right)$$s.

When we set $$\theta=0 \text { and } \theta=\pi$$

We get a singularity on one of the axises.

What is a quasi-regular singularity as opposed to a curvature singularity?