Effusion
Effusion: 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.
Image by James Heilman, MD - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=10429260