ordinary cap; field cap; garrison cap
A simple, often military-style cap without a brim or with a soft visor; used in casual or field settings. The term is somewhat dated and may be encountered in historical or military contexts.
The soldiers participated in training wearing their field caps.
From 略 (りゃく, 'abbreviated' or 'simplified') and 帽 (ぼう, 'cap'), referring to a simplified military cap.