"Horizontal abduction" refers to the movement of a body part away from the center of the body, specifically in a horizontal or sideways direction. Full definition
As you can see from the pictures above, the wide grip bench press involves a larger shoulder horizontal abduction angle than the close grip bench press. (strengthandconditioningresearch.com)
Reinold et al. (2004) found that external rotation exercises produced inferior middle deltoid muscle activity, while prone horizontal abduction with external rotation produced large (82 %) EMG activity. (strengthandconditioningresearch.com)
The largest muscle activity was found when performing horizontal abduction with internal rotation, followed by performing the exercise with external rotation. (strengthandconditioningresearch.com)