Short answers, pulled from the story.
Nominal rigidity, also called price stickiness or wage stickiness, describes a nominal price or wage that is slow to adjust or resistant to change. Complete rigidity holds a price fixed for a period, such as a contract fixing a good at $10 per unit for a year, while partial rigidity allows change within legal or institutional limits.
In The General Theory of Employment, Interest and Money, Keynes argued that nominal wages display downward rigidity because workers are reluctant to accept cuts in pay. He held that this reluctance produced involuntary unemployment, since wages take time to fall to an equilibrium level, a pattern he tied to the Great Depression.
Studies of Consumer Price Index microdata found that, once sales and temporary discounts are removed, complete price stickiness typically lasts around 12 months, with the average price spell length in the US more than doubling to 11 months and reference prices holding for an average of 14.5 months. In France and the UK, 19 percent of prices change in a typical month, implying an average spell of about 5.3 months.
The Taylor model, from John B. Taylor in 1980, sorts firms into cohorts that each reset prices on a fixed schedule, so a firm knows exactly how long its price will hold. The Calvo model, from Guillermo Calvo in 1983, instead gives every firm a constant probability h of resetting its price each period, so the length of a price spell is never known in advance.
Sticky information, proposed by Stanley Fischer in a 1977 article, holds that prices look sticky because the information behind them is old, not because the price itself resists change. Fischer modeled two unions taking turns setting wages for two periods at a time, so one union always acts on newer information than the other.
Sticky inflation is caused by expected inflation, such as home prices rising before a recession, wage-push inflation from a negotiated raise, and temporary inflation caused by taxes. Stagflation occurs when economic output decreases while inflation increases, a combination that makes the standard of living fall faster and that neither monetary expansion nor contraction reliably fixes.