osu! is a freeware rhythm game made by Dean "peppy" Herbert, based on the gameplay of the Ouendan and Elite Beat Agents series of games for the DS.Game levels, called beatmaps, are created and submitted by users via a Level Editor, refined through community modding and ultimately approved by members of the Modding Assistance and Beatmap Appreciation Teams.Besides the Ouendan/EBA gameplay, osu! includes three additional game modes: one based on the Taiko no Tatsujin series, one based around the gameplay of beatmania IIDX / O2jam and another revolving around catching fruit. Players can compete in all four modes in online multiplayer matches.Its website can be found here and the game can be downloaded here.
April Fools' Day: The jokes each year revolve around changes in the game's genre ("Touhosu", a Touhou/osu! hybrid), gameplay ("osu!core", where the songs in every beatmap had their pitch and speed increased), staff (unexpected demotions or promotions) and beatmaps (pretending to approve and praise an extremely poorly made beatmap
Also, some of those jokes have ascended. osu!core became a permanent available mod called Nightcore, and Touhosu! has been confirmed as in development by peppy.
Determinator: A player can become this if they're trying to pass, get an FC (Full Combo), or enter the Top 50 on a beatmap, especially if the beatmap is pretty difficult. If they lose a rank in the top 50, don't expect them to quit playing for hours until they beat the player who beat their score.
Fun with Acronyms: MAT (Modding Assistance Team), BAT (Beatmap Appreciation Team) and, until its merging into the BAT, GMT (Global Moderator Team).
Grade Inflation: As in Ouendan and EBA, S comes after A, but a perfect performance in osu! earns the player an SS rank. An SS achieved under the Hidden and/or Flashlight mods has a separate sprite (alternate colouration in the official skin) and commonly referred to as "SSH".
This especially applies for the beatmaps with >=4.95 Star Difficulty. And approval beatmaps with crazy streams/jumps. An example is this beatmap, based from "Can't Defeat Airman". It's a thing of the past, though; there's a new star difficulty calculation system as of March 2014.
A common joke between pro players (rank 1000 or less) is saying they have problems passing "Can't Defeat Airman", there are a few beatmaps where almost all players have to struggle, such as "xi - FREEDOM DiVE (FOUR DIMENSIONS)", or "IOSYS - Utage wa Eien ni ~SHD~ (TAG4)".
And then there are the few maps that completely break the star rating system, such as the already mentioned xi - FREEDOM DiVE, where the hardest difficulty is rated at around 2.5 stars, which is the average Easy difficulty rating, and the other, also insane+ difficulty is rated at 4.5, which is a typical Hard difficulty. Certain maps with high difficulty settings go ABOVE the 5.00 limit(although not by much).
Adding on, there are several joke maps with star ratings into the negative and beyond, even reaching up to -infinity/5 in some weird special cases.
Elaborating on the new calculaton system above: Anything that goes up to 3.75 stars (which is declared by the system as an Insane diff, symbolized with a red star and an I letter in the middle of it) can be considered this, but some Hard diffs (2.5-3.74 stars, with an orange star and an H letter), especially the ones on the higher scale can also be Nintendo Hard.
The hardest songs from Ouendan/EBA are approximately 3.5 stars on the new star rating scale. The hardest songs on Osu, such as "Can't Defeat Airman" exceed 7 stars.
One-Hit-Point Wonder: In the Sudden Death mod, you fail the beatmap you're currently playing if you screw up once.
With the "Perfect" mod, you lose instantly if your accuracy goes below 100% at any point.
Scoring Points: There are two scoring systems: the "PointValue x (combo + ConstantA) x (ConstantB)" system used in the source games, and the "pp" system (each map gives you a certain amount of pp if your performance is better than what was expected according to your current Overall pp Rank). There are also scoring system that are exclusive for multiplayer matches, such as accuracy (percent-based system, which is the average accuracy for each note, being a 300 a 100%, a 100 a 33.33%, a 50 a 16.66% and a MISS a 0%), and Combo (the player with the biggest combo the moment the map ends wins). In a multiplayer match, you can choose which system to use for ranking.
Self-Imposed Challenge: Using one or more of the difficulty increasing mods and/or going for SS (perfect) runs.