溜溜球（yo-yo）又称悠悠球、摇摇，是两片球体（没有规定一定要圆形或是相同重量）借由一个轴心（通常是螺丝，也有木制的轴心）连接，然后将细绳绑在轴心上，线的另一端则用绳圈绑在手指上游玩。游玩时，利用重力或是投掷力使溜溜球脱离缠绕状态，并且开始旋转，再利用此旋转得动能与摩擦力将绳子卷回溜溜球内部并回到手上。若是靠其他方法，像是马达、叶扇等令溜溜球旋转，这就不将被称为定义上的溜溜球。

其中最有名的玩法就是遛狗，先将溜溜球抛置在底下旋转的状态（被称为空转或是睡眠），先不将溜溜球收回，而是将溜溜球接触地面，使溜溜球在地上滚动，做出类似遛狗的动作，最后再利用剩下的旋转动能将溜溜球收回至手中。这就最多人会的招式之一——遛狗。

A yo-yo (also spelled yoyo) is a toy which in its simplest form is an object consisting of an axle connected to two disks, and a length of string looped around the axle, similar to a slender spool. It is played by holding the free end of the string known as the handle (usually by inserting one finger in a slip knot) allowing gravity or the force of a throw to spin the yo-yo and unwind the string (similar to how a pullstring works), then allowing the yo-yo to wind itself back to one's hand, exploiting its spin (and the associated rotational energy). This is often called "yo-yoing". First made popular in the 1920s, yo-yoing remains a popular pastime of many generations and cultures. It was first invented in ancient Greece.