Chord
A chord is a group of two or more notes that are hit at the same time. They are known to increase density and highlight different sounds within the music.
Jump
Also known as doubles, jumps are two notes pressed at the same time. This is the most common chord type in 4K osu!mania.
The term comes from Dance Dance Revolution and similar games where performing doubles requires the player to literally jump between corresponding arrows on the controller pad.
Hand
Also known as triples, hands are three notes pressed at the same time.
Quad and other chord sizes
Quads are four notes pressed at the same time. Past this amount of notes, chords are typically referred to numerically, such as "five-note chord", or "six-note chord". Larger chord sizes like these are more common in key modes higher than 4K.
Grace note
Grace notes are two or more notes in different columns that are meant to be played in rapid succession from one another. This is commonly done with snappings higher than 1/6. During gameplay, they appear very similar to chords.