The definition of the slow-wave factor

Could someone please show the definition of the "slow-wave factor", or simply offer a link to those corresponding references?
many thanks!

http://www.microwaves101.com/encyclopedia/slowwave.cfm

hi vfone,
thanks for your information!
according to its definition, the slow-wave factor also eaquals to β/k0, where β is the phase constant of the slow-wave structure and k0 is determined by the constitutive parameters of the media, and also eaquals to the index of refraction n.
I'm wondering if i've correctly understood it.

The propagation constant and phase velocity of a loss-less transmission line are given by β =2πf√LC and Vp = 1/√LC , where L and C are the inductance and capacitance per unit length along the transmission line.
Slow-wave propagation can be accomplished by effectively increasing the L and C values.
This can be done introducing periodic variations along the direction of propagation (drilling holes in the substrate, or by etching patterns in the microstrip ground plane).

yes, transmission lines with periodically loaded C and L are one typical slow-wave structure.
in one paper i'm examining at the moment, it seems the "slow-wave factor" is used to described any wave-guiding structure (even, e.g a parralel plate waveguide filled with dielectric material).
Thank you!