The Collaborative International Dictionary
Divisional planes
Divisional \Di*vi"sion*al\, a. That divides; pertaining to, making, or noting, a division; as, a divisional line; a divisional general; a divisional surgeon of police.
Divisional planes (Geol.), planes of separation between rock masses. They include joints.
Wiktionary
divisional planes
n. (plural of divisional plane English)