Streamline diffusion, given an advection -diffusion equation, refers to all diffusion going on along the advection direction.
Explanation
In an advection equation, for simplicity assume:
, and

Adding a diffusion term, again for simplicity, assume the diffusion to be constant over the entire field.
,
Giving an equation of the form:

The equation may now be rewritten in the following form:

The term below is called streamline diffusion.

Crosswind diffusion
Any diffusion orthogonal to the streamline diffusion is called crosswind diffusion, for us this becomes the term:
