Function
GMAT glossary · Verbal Reasoning
Definition
A rule that assigns exactly one output to each input in its domain.
Function in plain English
A function is a rule that takes an input, does something to it, and gives back exactly one output, no ambiguity allowed. Picture a vending machine: press button B4 and you always get the same snack; you'd never press one button and randomly get two different things. That 'one input, one output' promise is the whole point, and it's what lets us reliably predict results, like a rule that says 'double the input and add 1,' turning 3 into 7 every single time.
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