Pus-filled cavity in root of decaying or dead tooth, formed as bacteria infect and destroy pulp and produce toxins; tooth very tender when tapped, painful to chew on, with a persistent ache or throb, and pain sometimes radiating along side of nose or across other teeth; if left untreated, pus seeps out through root, erodes a canal through adjacent bone, and causes a gumboil, a painful swelling on the gum.