A Full Combo, commonly abbreviated to FC, is a concept found in music and rhythm games referring to the event where a player successfully hits all of the notes in a song in succession without breaking their combo chain or multiplier, usually indicated with an onscreen combo counter. The first official in-game usage of the term was from Dance Dance Revolution SuperNOVA 2, released in 2007, though the term was already in use before this point among rhythm game enthusiasts and arguably became an ingrained part of the English vernacular with the rise of the Guitar Hero franchise and the spread of Full Combo videos being shared on YouTube.
The exact definition of what constitutes a Full Combo differs from game to game as scoring systems across games differ though common across all games is the idea that a successful Full Combo on the hardest difficulty is generally regarded as a mark of skill.
In the Guitar Hero and Rock Band franchises, a Full Combo simply requires a player to hit all the notes in a song in succession without ever losing their multiplier. Extra strums between notes result in unsuccessful attempts due to the multiplier being lost as a result. It is thus possible to hit all the notes in the song, getting a 100% on the results screen, but fail at achieving a Full Combo.
Getting full combos while playing Rock Band drums is a controversial issue. While a full combo can happen by just not breaking combo, people have made it clear that full combos are generally only accepted if the player uses the correct overdrive path (or OD path). For example: A player could maintain a combo by skipping every drum fill throughout the whole song. Drum enthusiasts feel that that is cheating and shouldn’t be considered a full combo considering they are skipping a large amount of notes, and parts that are sometimes very difficult. Players call these paths Bootleg FC’s. More often than not, bootleg FCs are preformed to skip difficult parts in the song to maintain combo, at the expense of extra points that could be obtained by performing those sections. The correct OD path is called the optimal path. This is because it gives the player the most amount of points possible. When the correct OD path is still undetermined because of the newness of the song or the difficulty, generally most FC’s will be accepted as long as they are not blatantly skipping Drum fills repeatedly.
In Japanese-styles rhythm games such as Dance Dance Revolution or beatmania games, notes in a song are generally scored in relation to how accurately they were hit and are given ratings generally ranging from 'Perfect' to 'Bad' and finally 'Miss'. In most of these games the accuracy rating right before 'Miss', or in other words the worst accuracy rating players can achieve while still technically hitting the note, generally breaks the combo chain while anything above that counts toward the combo. Thus there can be situations similar to the Guitar Hero and Rock Band franchises where 100% of the notes are hit but a Full Combo is not achieved due to the combo chain being broken.
Moreover, in games with such accuracy rating systems, communities tend to additionally recognize 'Perfect Full Combos' achieved by hitting every note in the song with the highest accuracy rating and maintaining a combo chain throughout. This is significantly harder to do due to the vastly decreased margin of error in timing.