the.com/shower door

means A hinged or sliding glass (or plastic) panel that encloses a shower to keep water in the stall instead of flooding the bathroom floor.

from The phrase is just honest compound English — 'shower' (from Old English 'scur,' meaning a brief fall of rain) plus 'door,' bolted together once indoor plumbing made standing under falling water a household habit rather than a weather event. The bathing sense of 'shower' is a 19th-century development of that old rain word, and the door followed naturally as soon as people started building stalls to contain the spray.