My Quantum Mechanics textbook (John S. Townsend's A Modern Approach to Quantum Mechanics) mentions regular solutions and irregular solutions. It claims that regular solutions (at the origin) to the spherical Bessel equation are called spherical Bessel functions, while its irregular solutions (at the origin) are called spherical Neumann functions.
What's the difference between "regular" and "irregular"? And, if possible, please also tell me what "regular" and "irregular" solutions are in general (i.e.: please don't just tell me how they differ without illuminating what they are in the first place).