Day Count Convention
Also called: act/360, 30/360, day count basis
A day count convention is the rule for turning a period between two dates into a fraction of a year — 30/360, actual/360, actual/365, actual/actual. It sounds like bookkeeping trivia and is worth real money: actual/360 pays roughly 1.4% more interest than actual/365 on the same quoted rate, because it counts 365 days over a 360-day year. Different markets use different conventions, so comparing quoted rates across them requires converting first.