(n.) light shining through the window of a house after dark
The light that can be seen shining through the window of a building from outside is called cotlight.
Etymologically, there’s little to report here: cot has been used as a word for a small, typically rural dwelling since the early Old English period; the more familiar word today, cottage, is a later development, showing the influence of the French suffix –age, and dates from the Middle English period. The purpose of –age here is debatable, but it’s likely the original implication of cottage was the entire plot of land on which a cot was located.
The word cotlight, meanwhile, dates from the eighteenth century, at least. The English Dialect Dictionary pinpoints it to the Lothian dialect of Scots English in particular—case in point, this verse from the four-part Scots poem The Waes o’ War, or the Upshot o’ the History o’ Will and Jean (1796), by the Midlothian-born writer Hector Macneill:
What’s this life, sae wae and wearie,
If hope’s brightn’ing beams should fail?
See! though night comes, dark and eerie,
Yon sma’ cot-light cheers the dale!