Short answers, pulled from the story.
Keynesian economics is a group of macroeconomic theories built on the idea that aggregate demand, the total spending in an economy, strongly influences economic output and inflation. It was developed by British economist John Maynard Keynes, whose 1936 book The General Theory of Employment, Interest and Money laid out the core framework during the Great Depression.
Keynes argued that markets do not automatically maintain full employment, and that when aggregate demand falls short of an economy's productive capacity, governments must intervene through fiscal and monetary policy. He wrote The General Theory during the Great Depression, when unemployment reached 25% in the United States and as high as 33% in some countries.
The Keynesian multiplier is the ratio between an increment of investment and the corresponding increment of aggregate income, reflecting how a single act of spending ripples through an economy in successive rounds. The concept was formalised by Richard Kahn in his 1931 paper The relation of home investment to unemployment, with the term multiplier itself suggested by Keynes.
The oil shock of 1973 produced stagflation, a simultaneous rise in unemployment and inflation that contradicted the Phillips curve's prediction and could not be addressed by a single Keynesian policy response. This discredited Keynesian near-consensus and enabled the rise of monetarism, supply-side economics, and new classical macroeconomics.
The Keynes Plan was a proposal Keynes put forward at the 1944 Bretton Woods conference for an International Clearing Union that would issue a new international currency called the bancor, exchangeable with national currencies at fixed rates. The plan would have taxed surplus nations to eliminate trade imbalances, but was rejected in part because American opinion resisted equal treatment of creditors and debtors.
A liquidity trap occurs when interest rates approach their lower limit and monetary expansion ceases to stimulate investment or demand, making monetary policy ineffective. Paul Krugman applied the concept to Japan's prolonged stagnation around the turn of the millennium, when short-term interest rates were near zero yet private investment remained insufficient to end deflation.