[ 'klɪəˌstɔ:ri ]
( US also clearstory )
■ noun ( plural clerestories )
1》 the upper part of the nave, choir, and transepts of a large church, containing a series of windows.
2》 a raised section of roof running down the centre of a railway carriage, with small windows or ventilators.
Origin
ME: from clear + storey .