Aid

From Fire Emblem Wiki

Aid is a stat which has made appearances in many games throughout the series, but has frequently gone by other names, or modified parameters. A basic summary of its function, is a unit's ability to rescue another unit. The highest aid stats are usually attributed to mounted units, such as cavaliers and wyvern riders. A unit's success in rescuing another unit is frequently evaluated by comparing the aid of the rescuer with the constitution or build of the unit to be rescued.

In most infantry units, the aid stat is one less than the unit's constitution. In mounted units, however, it is a set value (e.g. 20, 25) minus the constitution.