Fortnight
What it really means
Two weeks. A normal everyday word in Britain (not slang, but it confuses many learners because Americans rarely use it). "See you in a fortnight" = see you in two weeks.
Context, tips & common mistakes
A fortnight is exactly two weeks, and unlike most 'quaint' vocabulary lists suggest, it is completely standard living English here — rent is quoted fortnightly, holidays booked for a fortnight, appointments made 'a fortnight on Tuesday' (two weeks from this coming Tuesday). Americans famously do not use it; Brits use it daily without noticing.
Adopt it for comprehension first: when a landlord says the boiler part arrives in a fortnight or payroll runs fortnightly, you now know your dates. 'A fortnight on Tuesday'-style constructions are worth decoding carefully the first few times — count from the next occurrence of that weekday, then add a week.