In singing, “accompaniment” refers to one or more parts that are played or sung, the intent of which is to support the solo but not to overpower.
In organ playing, an “accompaniment” (yellow highlight) refers to a stop or combination played on a manual other than that of the solo (and usually with pedal) that is supportive but less prominent than the solo. Specifically, the accompaniment is softer, less bright, or less colorful than the accompaniment.