That's a little like asking what is "The Moon" :^). Alternatively, like trying to discern a name for that squiggle that TAFKAP now (once?) use(s|d?).

Personally I've always referred to it verbally as "dollar-underscore".

Though, is the same way as the Moon can be referred to using several synonyms--Luna, the Earth's (only?) natural satellite, "heartless orb that rules night and steals the colors from our sight"--so I similarly think of it in different terms depending upon the context.

E.g. default value, default iterator, default argument etc.

Or simply "The default", though that only makes any sense with more context.