In the process of implementing a new computer language, the point at which the language is sufficiently effective that one can implement the language in itself.
That is, for a new language called, hypothetically, FOOGOL, one has reached break-even when one can write a demonstration compiler for FOOGOL in FOOGOL, discard the original implementation language, and thereafter use working versions of FOOGOL to develop newer ones.
This is an important milestone.
See My Favourite Toy Language
[There actually is a language called Foogol