The Love numbers (or Love-Shida numbers) are three numbers that describe the rigidity of a body, aimed at quantitatively describing the body's potential deformation due to a tidal force. They are scaled so 0 indicates "absolute rigidity", i.e., such a force would cause no deformation. The three Love numbers are:
h is scaled so that for a body with no rigidity, e.g., liquid, h-k would be 1. It happens that for such a body, k would be 1.5 and h would be 2.5. The numbers are unlikely to be uniform throughout an entire body, which can be made of many substances with differing characteristics. Modeling the numbers as varying over the surface using a multipole expansion, the numbers h2, k2, and l2 are the most interesting, the major reason being the symmetry of the nearly-spherical bodies (c.f., J2).
The numbers are used for the calculation of tides on the Earth, which was the motivation for their development. They are also of interest in astrophysics in studying the effects of tides on solar system moons, which offer a window into the moons' internal structure.