Lost Decade is a term that refers to the period after an economic and/or stock market collapse.
After a major collapse, the economy improves very slowly or not at all. The market goes nowhere...the same price a decade later versus when the collapse hit. The Dow hovered around 1,000 for a decade in the late 1960s to the late 1970s; it was a "lost" decade in a sense, but it wasn't like the world was standing still. During that period, the P/E ratio of the market went from something like 20x to something like 10x (i.e., the same companies were selling for half the price as they grew operating earnings, but market sentiment just turned negative). And dividends went from something like 4% to 7%.
So that "lost" decade of the Jimmy Carter '70s was, in fact, not so lost.