"true macro" so far as photography is concerned has, in my experiences and reading, alwasy been taken to be a 1:1 ratio of Size of the image as reflected on the sensor/film by the lens : size of the subject in real life. This ratio is also called actual size or life size.
Macro has also come to be used (by companies and people in casual conversation) to also mean "close up" photography of a ratio typically around 0.5:1 through to 1:1 (ie half life size to life size). In addition macro is also used as a catch all term for higher magnifications as well. Subject also comes into play - dragonflies, butterflies, moths and flowers are a good series of examples of photos that are often taken at weaker magnifications than 1:1 but which are generally considered "macro" as subjects.