Not to be confused with Beatmap.
A beatmap's difficulty is a game level, which is stored as a text file with the
.osu extension. It consists of hit objects of a certain game mode, which are set up accordingly to the timing tailored to a specific song. Difficulties have different aspects, which directly affect user experience. The difficulty name and star rating usually give enough information to tell how challenging it is.
The hit objects of a difficulty compose patterns, which also vary per game mode and set the tone for overall look and feel of how a difficulty should be played. The gameplay of beatmap difficulties can also be altered by various difficulty settings set by the beatmap's author or toggled by game modifiers.
From the auditory perspective, a difficulty can augment the song by providing one or multiple sets of audio samples, or hitsounds, that serve as an additional layer of feedback.
Visual components of a difficulty, contrary to its patterns, provide auxiliary experience during gameplay and can be disabled in the visual settings overlay for the most part. While some of them, such as storyboards or custom skins, belong to the beatmap as a whole, most of the visual components may vary per difficulty (e.g., kiai time sections and combo colours).
See also: Difficulty naming
These are the typical levels of difficulty that a beatmap can fall under. Specific definitions and requirements of each can be found in their respective mode's ranking criteria.
As of 27 July 2021, the osu! website automatically assigns a beatmap's difficulty rating colour based on their star rating according to the following spectrum:
In some instances, such as AIMod, difficulty is classified into six level ranges based on star rating as follows:
Note that while the intended difficulty levels of most beatmaps match the colours or level ranges they are given, some may deviate significantly due to certain factors. This is why the ranking criteria offers more decisive ground rules to determine a beatmap's actual difficulty level and spread regardless of its star rating.