The seeping into, or abnormal collection of fluid in a body cavity. For example, knee effusion describes swelling within the knee joint. Note: the word "effusion" is used differently in medicine than it is used in reference to the movement (seeping out) of liquids and gases, for example in chemistry.  

Related Terms

  • Joint effusion
  • Pleural effusion
  • Ascites
  • Pericardial effusion


  • Fluid accumulation
  • Fluid buildup
  • Swelling
  • Edema
  • effused
  • Effusion
  • effuse
  • effuses