Etch rate r:
The etch rate is the abrasion per time and for example can be specified in nanometers per minute or Angstrom per second.
$$r=\frac{Removal \Delta z}{Etch time \Delta t}$$
Anisotropism f
The anisotropism gives the ratio of horizontal etch rate rh and vertical etch rate rv.
$$f=1-\frac{r_h}{r_v}$$
For structuring high anisotropic processes are desired, which means an etching only in vertical direction, so that the resist mask is not underetched. For anisotropic etch processes f → 1, and accordantly for isotropic processes f → 0.
Selectivity Sjk
The selectivity Sjkof layer j and layer k is the ratio of the etch rates of two films, e.g. of the layer which should be structured (j; e.g. oxide) and the layer which should not (k; e.g. resist).
$$S_{jk}=\frac{r_j}{r_k}$$
The value of the selectivity depends on the process. For structuring the value should be as high as possible, which means that the layer one wants to structure is etched much faster than a masking resist layer. In reflow back etching the selectivity should be about 1 to ensure a uniform surface.