A unit of account is the measuring stick prices are quoted in.
Why it sounds harder than it is
Grams for weight, litres for milk, dollars for value.
The phrase sounds like it belongs in a ledger because it was borrowed from one. What it describes is ordinary: one shared measure, so two unlike things can be compared.
The useful part is that you can hold a coffee, a car and a year of rent in your head at once, because all three arrive in the same unit. That is so normal it never gets named, and the moment someone names it formally it sounds like a subject you missed.
A concrete Australian example
A supermarket shelf label carries two numbers: the price, and the price per 100 grams.
Both are in dollars. The dollars are doing the measuring, so you can put a large packet next to a small one and tell which is dearer.
Your rent, your phone bill, your HECS balance and your super all arrive quoted in the same unit. They are not comparable because they are similar. They are comparable because they are measured the same way.
What it is not
- Not the same as a medium of exchange. One measures, the other pays. A currency usually does both jobs, and they can come apart: prices are sometimes quoted in one currency and settled in another
- Not fixed. The stick itself changes length over time. That change is what inflation describes
- Not the same as value. A price is a measurement of value at a moment, taken with a stick that moves
Related
The measuring stick is worth a second look whenever a price is quoted per week and the contract runs for a year.