Refers to a period of strong economic activity or general prosperity. Often used in economic contexts.
After the war, Japan enjoyed a boom.
This area is prosperous thanks to tourism.
不景気 means 'recession' or 'economic slump', the opposite of 上景気.
Compound of 上 (じょう, 'upper' or 'high') and 景気 (けいき, 'economic conditions'), literally 'high economic conditions'.