What is a stiff lung?

Inflation problem: This usually means there is a problem with the lungs expanding properly. There is some sort of restriction to keep the lungs from inflating.

Several things: It can refer to several issues. Most classically it refers to ARDS(acute respiratory distress syndrome) which occurs when people develop wide spread inflammation in the lungs, which causes the lungs to become stiff and noncompliant and leaky. But other processes that lead to "interstitial lung disease" can also make the lungs stiff and mechanically not as "flexible".