The difference between oedema and effusion
is that "oedema" is an excessive accumulation of serum in tissue spaces or a body cavity and "effusion" is a liquid outpouring.
The difference between oedema and ascites
is that "oedema" is an excessive accumulation of serum in tissue spaces or a body cavity and "ascites" is an accumulation of fluid in the peritoneal cavity, frequently symptomatic of liver disease.