1.
one shot, one blow
A single shot (gun), punch, or strike.
2.
one try, in one attempt
Doing or achieving something in a single attempt.
一発 relates to single actions with impact.
PHYSICAL ACTION (Sense 1):
SINGLE ATTEMPT (Sense 2):
BIG SUCCESS (Sense 3):
Mahjong
- 一発: a bonus for winning within one turn after riichi