Rayleigh Scattering
Scattering by refractive index fluctuations (inhomogeneities in material density or composition) that are small with respect to wavelength. 

Refractive Index Profile
The refractive index measured in a fibre as a function of the distance from the axial core or center.

Refraction Index
The ratio of light velocity in a vacuum to its velocity in the transmitting medium.

Return loss (RL)
Power ratio of transmitted signal to reflected signal of an optical port. 

Ribbon
Multiple fibres organized in a flat array.