一下 has three meanings as follows ◎ 一下 yíxià [one time;once] 亲一下孩子的脸蛋（kiss the child on the cheek） ◎ 一下 yíxià [in a short while]∶ eg:灯一下亮了 the light turned on immidiately. 我等一下去你家 I'll go to your place very soon ◎一下 [in one go;at a single stroke;all of a sudden]∶ 天一下黑了下来 It turned dark all of a sudden

一下is similar to little.for example, 等一下 means "wait for little moment" .But 一下 is an adv.

when you pick up the phone, someone want to talk to other one, you may say one moment （等一下) Other example: I will arrive your home immediately. 我再过一下就到你家了.

一下 has three meanings as follows ◎ 一下 yíxià [one time;once] 亲一下孩子的脸蛋（kiss the child on the cheek） ◎ 一下 yíxià [in a short while]∶ eg:灯一下亮了 the light turned on immidiately. 我等一下去你家 I'll go to your place very soon ◎一下 [in one go;at a single stroke;all of a sudden]∶ 天一下黑了下来 It turned dark all of a sudden