Double time

In popular music, half-time is a type of meter and tempo that alters the rhythmic feel by essentially doubling the tempo resolution or metric division/level in comparison to common-time. Thus 4
4 approximates 8
8. It is not to be confused with alla breve or odd time. Though notes usually get the same value relative to the tempo, the way the beats are divided is altered. While much music typically has a backbeat on quarter note (crotchet) beats two and four, half time would increase the interval between backbeats to double, thus making it hit on beats three and seven (count out of an 8 beat measure [bar], common practice in half time):

Essentially, a half time 'groove' is one that expands one measure over the course of two. The length of each note is doubled while its frequency is halved).

Time signatures are defined by how they divide the measure (in 9/8, complex triple time, each measure is divided in three, each of which is divided into three eighth notes: 3*3=9). In "common"-time, often considered 4/4, each level is divided in two (simple duple time: 2*2=4). In a common-time rock drum pattern each measure (a whole note) is divided in two by the bass drum (half note), each half is divided in two by the snare drum (quarter note, collectively the bass and snare divide the measure into four), and each quarter note is divided in two by a ride pattern (eighth note). "Half"-time refers to halving this division (divide each measure into quarter notes with the ride pattern), while "double"-time refers to doubling this division (divide each measure into sixteenth notes with the ride pattern).

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Wikipedia