As others have noted, this could be described in basic terms as sitting with sitting with crossed legs or with a leg crossed, but to be more specific, Allan Pease's Body Language: How to read others' thoughts by their gestures (Sheldon Press, 1981) refers to this specific position as an "American figure four" because it is supposedly a

sitting position used by many American males who have a competitive nature

and to be contrasted with what he terms the "standard" leg cross, as such:

He retains this terminology in The Definitive Book of Body Language (Random House, 2008):

in contrast with the Europea Leg Cross (with a higher knee) and the Ankle Cross among others, and while I have never heard those terms in everyday use, I have heard of one crossing his legs in a figure-four. Whether originating from these books or from elsewhere, the terminology is used not only by other body language websites and books but in medical uses and massage guides, among others.

You probably mean sitting with leg crossed: (ankle-on-knee)

• figure sitting position involves resting one foot over the thigh of the other, and it's usually much more comfortable for men than the tight leg cross. It’s a popular male leg cross, although it can be seen in some assertive and extrovert women too.

commons.wikimedia has loads of such pictures in...

Category:Sitting with legs crossed (ankle-on-knee)