Definition from Wiktionary, the free dictionary
Alternative forms 
Contraction of good day.
- IPA: /ɡəˈdeɪ/
- (Australia, New Zealand, colloquial) Hi, hello.
- (North America, air traffic control, informal) Denotes the end of a radio transmission.
Usage notes 
- (good day): Used interchangeably with hello and hi, but more characteristically (and stereotypically) Australian/New Zealand, and perhaps the most informal of these options. Also used in the constructions G′day, mate (a greeting to a friend or acquaintance) and G′day, stranger (ironically, to a friend not seen in some time).