Unrecorded and untaxed dealings

Wiktionary Word definitions in Wiktionary
n. (context economics English) A sector of a nation's economy that is illegally undocumented due to cash payments or tax avoidance

Longman Dictionary of Contemporary English Word definitions in Longman Dictionary of Contemporary English
noun EXAMPLES FROM CORPUS ▪ Boosts the black economy , I suppose. ▪ If taxes rise simultaneously there is the prospect of a major disincentive effect on output or of growth in the black economy . ▪ It leads to the stimulation of a privately run black economy ...

WordNet Word definitions in WordNet
n. a hidden sector of the economy where private cash transactions go unreported; "no one knows how large the black economy really is"