Exponential describes a very rapid increase. An exponential rise in the number of first graders with head lice might be a result of all the hat sharing that's gone on lately.

Exponential is also a mathematical term, meaning "involving an exponent." When you raise a number to the tenth power, for example, that's an exponential increase in that number. When your next door neighbor describes the exponential growth of her rose bushes or the exponential increase in members of her book group, she means that there's been a sharp, fast expansion of both things.

Definitions of exponential
  1. noun
    a function in which an independent variable appears as an exponent
    synonyms: exponential function
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    type of:
    function, map, mapping, mathematical function, single-valued function
    (mathematics) a mathematical relation such that each element of a given set (the domain of the function) is associated with an element of another set (the range of the function)
  2. adjective
    of or involving exponents
    exponential growth”
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