fine; penalty fee
Specifically a monetary penalty for minor legal violations, such as traffic or parking infractions, distinct from criminal fines (罰金).
駐車違反で反則金を払った。
I paid a fine for a parking violation.
スピード違反の反則金はいくらですか。
How much is the fine for speeding?
Compound of 反則 (violation of rules) and 金 (money).