a rounded swelling

Inflammatory swelling of one or more lymph nodes, usually in the groin; the confluent mass of nodes usually suppurates and drains pus.

A bubo is a swelling of the lymph nodes, found in an infection such as bubonic plague, gonorrhea, tuberculosis or syphilis. According to historical records they were also characteristic of the pandemic responsible for the Black Death and perhaps other ancient pandemics. It usually appears under the armpit, in the groin or on the neck.