Which condition is characterized by the presence of fluid in the peritoneal cavity?

Ascites refers specifically to the accumulation of fluid within the peritoneal cavity, which is the space between the abdominal organs and the abdominal wall. This condition can arise due to various factors such as liver disease, heart failure, infections, or malignancies. The presence of this excess fluid can lead to abdominal distension and discomfort, and it may require diagnostic procedures like ultrasound for evaluation and management.

The other conditions listed pertain to different areas: pericarditis involves inflammation of the pericardium surrounding the heart, pneumothorax refers to air in the pleural space affecting the lungs, and cholecystitis is the inflammation of the gallbladder. Therefore, only ascites is directly related to fluid accumulation in the abdominal cavity.