How easily a piece of work could be handed to another person who would do it just as well, just as soon.
Example
A garden centre pays $9 an hour to water the plant beds and $22 an hour to repair the irrigation system. Watering can be taught in a morning. The repair skill takes years, so far fewer people can do it.
How it fits in
Replaceability describes the work, never the worth of the person doing it. Three things make work hard to replace: skill that took years to build, judgement where there is no obvious right answer, and responsibility when a mistake is expensive. Plenty of work a country could not run without is easy to replace and paid poorly, and that gap is a real problem rather than a law of nature.
Where this is taught
Related terms
When the money you earn from saving starts earning its own money on top.
Interest paid only on the original amount, never on the interest you have already earned.
Needs are the things that keep you safe and well. Wants make life nicer. Mixing them up is what empties most budgets.