The tang is the part of the fretwire that seats down into the slot cut in the fretboard. It is usually machined with raised barbs that help hold it in place when seated. The depth and width of the tang can vary from fret wire to fret wire, but most modern fret wires are made with a tang that will fit well into a slot width about .023″ wide.
Synonyms:
fret tang, fret wire tang, fretwire tang, tang depth, tang height