What it means
An ending that marks a singular noun in words borrowed from Latin: a bronchus, a thrombus, an embolus, a fundus, a fetus. The plural usually changes -us to -i: bronchi, thrombi, emboli. A few, such as fetus and sinus, simply add -es.
Dictionary · Word part
-us
A Latin noun ending found on many body structures and things, such as bronchus and thrombus.