In _Robbins Basic Pathology 9th ed., edema is defined as
[E]dema is an accumulation of interstitial fluid within tissues. Extravascular fluid can also collect in body cavities such as the pleural cavity (hydrothorax), the pericardial cavity (hydropericardium), or the peritoneal cavity (hydroperitoneum, or ascites).
Though hydrothorax is mentioned here in relation to edema, it is not mentioned explicitly whether it is actually a subtype of edema or not. I also looked into multiple sources for edema definition; “accumulation of fluid in the interstitum”; interstitium refers to the tiny spaces between cells which is not the case for hydrothorax since pleural cavity is not interstitium (right?). So is hydrothorax considered as a subtype of edema?