IMO: International Mathematics Olympiad.
The International Mathematical Olympiad (IMO) is the World Championship Mathematics Competition for High School students and is held annually in a different country. The first IMO was held in 1959 in Romania, with 7 countries participating. It has gradually expanded to over 100 countries from 5 continents.
The competition involves teams of up to six contestants sitting two exams on consecutive days. Each exam lasts 4.5 hours and contains three hard problems. Contestants’ scripts are marked over the next few days with each solution (or attempt) being awarded a score out of seven. Gold, Silver and Bronze medals are awarded to roughly half of the contestants in the ratio 1:2:3. Those not within the medal boundaries, but achieving full marks on at least one problem, are awarded an Honourable Mention.
All information about the IMO you can find on: https://www.imo-official.org/