n. — especially Ontario & Yukon, Social customs
a party held at night in a wooded area, usually by young people, to avoid parental or police control of the consumption of intoxicants.
Type: 5. Frequency — The general meaning of 'bush' as a woodland (see OED-3, s.v. "bush" (9a)) seems to be an adaptation of the Dutch bosch "in colonies originally Dutch" and is not limited to Canada (see OED-3, s.v. "bush" (9a)). As Chart 1 shows, the term is more frequent in Canada than in the US or the UK. Within Canada, the term is most frequent in Ontario (and Yukon) (see Chart 2).
For 'bush', see also COD-2, s.v. "bush party", which marks the term "Cdn".