from Tanglewreck:

'What's a mistress?' asked Silver.

Micah hesitated. 'A mistress be the woman you love even though you not be married to her...The whole of London be digged with such Lady Lanes, so that a man may travel in secret to his mistress. Some be deep, some be shallow, some be creeping for miles through the gloom of the pit, some be connecting two houses that rubs next to one another, and all lit by love.'