Compound metre (or compound time), is a metre in which each beat of the bar divides naturally into three equal parts. That is, each beat contains a triple pulse (Latham 2002a). The top number in the time signature will be 6, 9, 12, 15, 18, 24, etc.

Compound metres are written with a time signature that shows the number of divisions of beats in each bar as opposed to the number of beats.

https://en.wikipedia.org/wiki/Metre_(music)