Ah I've resolved my long-standing question about this phrase. All this time I thought it was only used like in mathematics, but not so! https://www.lexico.com/

Ah I've resolved my long-standing question about this phrase. All this time I thought it was only used like in mathematics, but not so! https://www.lexico.com/definition/by_definition https://www.lexico.com/definition/by_definition