Version August 2025
| Term | Definition |
| Parapneumonic effusion | Fluid in the pleural space in the setting of an adjacent pneumonia |
| Uncomplicated (simple) parapneumonic effusion | Free-flowing effusion that is sterile |
| Complicated parapneumonic effusion | Effusion infected with bacteria or other micro-organisms or having biochemical properties suggestive of recent infection |
| Empyema | Pus in the pleural space (from pneumonia or other source) |
| Complex effusion | Effusion with internal loculations (septae) |
| Uniloculated effusion | Effusion that is without internal septae (free-flowing or fixed) |
