Pronunciation: Brit. /ˌləʊˌsʌɪd ˈwɪndəʊ/, U.S. /ˌloʊˌsaɪd ˈwɪndoʊ/
Etymology: < low adj. + side n.1 + window n.
Church Archit.
[‘ In some medieval churches: a small window lower than the other windows and typically situated in the chancel or presbytery. It is the space around the altar in the sanctuary at the liturgical east end of a traditional Christian church building, possibly including the choir.[1] It may terminate in an apse. or arch.’]
The purpose of these windows is uncertain and has been the subject of much debate; they were popularly believed to provide a means by which lepers could view the altar (cf. leper window n. at leper n.2 ).
- 1825 T. Rickman Attempt to discriminate Styles Archit. Eng. (ed. 3) 215 The curious long window reaching lower down than the others..seems to have had some purpose of giving light behind the screen dividing the nave and chancel. It..may be called a low side window for distinction, as it is always lower down than the general range of the chancel windows.
- 1848 Ecclesiologist 8 375 Where neither low side window nor bell-cot existed.
- 1894 Murray’s Handbk. Oxfordsh. 103 On the S., is a low side window, that is blocked.
- 1938 Times 4 Oct. 10/4 May I suggest that the object of the low side window was a purely practical one: the provision of ventilation and..additional light.
- 1954 N. Pevsner Cambridgeshire 209 Neither their purpose nor their connexion with the often mentioned so-called low-side-windows nor their evolution have been sufficiently studied.
- 2007 S. Roffey Medieval Chantry Chapel v. 50 Interestingly, the ‘low-side’ window at Britford is situated in the west wall of the south transept.


from Oxford English Dictionary.