The definition I've heard is when your subject is reproduced on your recording medium (film or digital sensor) near or at 1:1 or better. So if your subject is 5mm long, it's near or at 5mm long on your recording medium. Or possibly more, so if it's 2:1 magnification, it's 10mm long instead of 5mm.
That’s the definition that I’ve always heard as well. Although personally I would open it up slightly with a less formalized definition: I find when the subject in a photo is bigger than it is in real life, when displayed at its intended size and viewing position, that it takes on a different dimension. I’m seeing something with more detail and from a different perspective than I can see with my own eyes. For lack of a more suitable term, I call that macro photography. Obviously if you project something on a large screen or zoom in 100% you can have it larger than life, but that’s not its intended size and viewing position.