fenestella
Concise Oxford English Dictionary © 2008 Oxford University Press:
fenestella /ˌfɛnɪˈstɛlə/
▶noun Architecture a niche in a wall south of a church's altar, holding the piscina and often the credence.
– origin ME: from L., dimin. of fenestra ‘window’.
'fenestella' also found in these Oxford entries:

