In a true balanced line situation, the inverted output is generated with respect to the non-inverted output and vice versa, rather than ground.
With all due respect to Ralph, I am going to quibble with this definition. There's more than one way to design a balanced circuit and operational amplifiers have long been used to generate both halves of the signal. There are advantages to the AES48 standard, but I don't think it's accurate to say that only amplifiers meeting the standard are "truly balanced."
By way of example, most (if not all) ARC amplifiers are differentially balanced yet don't comply with AES48. To claim that all ARC did to achieve balanced outputs is "add a 2nd output ... out of phase with the first" really doesn't accurately describe the amplifier's circuit.