Effusion is defined as:

Effusion is defined as the escape of fluid into a cavity, making it the correct choice. This term is commonly used in medical contexts to describe when excess fluid accumulates in spaces within the body, such as joints (joint effusion), pleural space (pleural effusion), or pericardium (pericardial effusion). This process can occur due to various conditions, including inflammation, infection, or injury.

Understanding the context of effusion is crucial in diagnosing and treating underlying conditions that lead to fluid accumulation. The other terms listed do not accurately capture the definition of effusion. Painful urination refers to dysuria, obstruction of a blood vessel pertains to thrombosis or embolism, and severe fatigue does not relate to fluid dynamics within body cavities. Thus, the essence of effusion centers on fluid movement, making it a key concept in medical terminology.