Answer: Lipohemarthrosis (notice the fat-fluid level which is oriented vertically on this Cross-table lateral knee)

Reference: http://radiopaedia.org/articles/lipohaemarthrosis

Interesting notes:

Ideally the patient has been lying in that position for 5 or so minutes to allow the fat and blood to adequately separate

It is important to remember that up to 64% of tibial plateau fractures do not have a x-ray visible lipohaemarthrosis, but rather a simple haemarthrosis