soft soap; potash soap
A type of soap made with potassium hydroxide, often liquid or semi-solid. Contrasts with カリ石鹸 (potash soap) in some contexts, but here specifically refers to soft soap, especially potash-based.
See also: カリ石鹸
Soft soap contains potassium salts, so it is softer than bar soap.
固形石けん is hard bar soap, typically made with sodium hydroxide, contrasting with the soft texture of 軟石けん.
Standard modern spelling using けん for 鹸.
Traditional kanji spelling; still seen but less common than 軟石けん.
Compound of 軟 (nan, 'soft') and 石けん (sekken, 'soap'). The term directly describes the physical property of the soap.