What Exactly Is a “Default”?

There are at least two senses in which the term "default" gets used: "Any failure by the United States to make any payment in full and on time" Volokh Conspiracy: Some of this reflects garden-variety misunderstanding, but some of it reflects a kind of intentional slipperiness.

He says a default on Treasury securities is terrible because the market for Treasuries is an important part of the global financial system.

The potential scale of spending delays in the weeks that come is vast, but it's hardly "unprecedented" for government payments to be delayed.

The degree to which spending delays will be damaging will depend on how far past X Date we go.

The mismatch between Treasury's available incoming cash and each day's thousands of required payments will continue to grow.

Short delays will eventually become long delays in payments, and the case will look less and less like a short government shutdown.