exponential: literal and figurative meanings
A rate that multiplies vs. a loose word for fast.
Exponential is a mathematical term for growth by a constant factor: 2, 4, 8, 16. A common error is to use exponential to mean very fast, as in The line at the counter grew exponentially. For a fast rise without a constant factor, use sharp, rapid, or steep: a sharp increase in fares.
Examples
UseThe bacteria grew exponentially, doubling every hour. (growth by a constant factor)
AvoidPrices rose exponentially last month. (if you mean only that they rose quickly)
Dictionaries now also list exponential meaning “very fast.” However, many people still consider this use an error, and they judge the speaker who uses exponential this way.