idempotent
/ eye-DEM-po-tent /
An operation is idempotent if doing it once and doing it ten times leave you in exactly the same place. Pressing the elevator button again doesn't summon ten elevators; the button is already lit, and pressing it changes nothing. The result is the same no matter how many times you repeat the action.
Contrast that with 'add 1 to my balance' — run it five times and you're five dollars richer, which is decidedly NOT idempotent. But 'set my balance to 100' is: run it once or fifty times, the balance is 100. The difference is whether repeating piles up effects or just lands on the same answer.
This is what makes retries safe. Networks drop, requests time out, and you often can't tell whether a request actually went through. If the operation is idempotent, you can simply try again without fear of charging the card twice or creating two copies — repeating it is harmless.
PUT /users/42 { "name": "Ada" } # set name to Ada — same every time
POST /users { "name": "Ada" } # create a user — runs again, makes a duplicatePUT sets a value (idempotent); POST creates a new thing each time (not).
The word comes from math: idem ('same') + potent ('power') — an action that, applied again, has the same power as once.