【见行堂语】

三摩地者，谓心专注所缘，复须于所缘相续而住。此须二种方便：一、於根本所缘令心不散，二、於已散未散将散不散如实了知。初即正念，次是正知。

Here, concentration refers to your attention remaining one-pointedly focused on an object of meditation;

in addition it must stay with the object continuously.

 Two things are needed for this:

(1) a technique in which your attention is not distracted from whatever it had as its original object of meditation, and (2) an accurate awareness of whether you are distracted and whether you are becoming distracted. The former is mindfulness; the latter is vigilance.