What is the exact difference between samatha and passadha? What is the exact difference between samatha and passadha?
samatha is a term frequently used. No problem defining that.

Passadha is not easily defined by web search. It appears and Portuguese and greek.

here is a link referring to passadha referring to the eye center.
http://www.dhammastudygroup.org/msg/125kd.txt

Perhaps there are other terms for passadha that are more widely used.