from m-w.com:
Main Entry: ex·trap·o·late
Pronunciation: ik-‘stra-p&-“lAt
Function: verb
Inflected Form(s): -lat·ed; -lat·ing
Etymology: Latin extra outside + English -polate (as in interpolate)
Date: 1874

transitive senses:

  1. to infer (values of a variable in an unobserved interval) from values
    within an already observed interval
    1. to project, extend, or expand (known data or
      experience) into an
      area not known or experienced so as to arrive at a usually conjectural knowledge
      of the unknown area <extrapolates present trends to construct an image
      of the future>
    2. to predict by projecting past experience or known
      data <extrapolate public sentiment on one issue from known public
      reaction on others>

intransitive senses:
to perform the act or process of extrapolating
– ex·trap·o·la·tion /-“stra-p&-‘lA-sh&n/ noun
– ex·trap·o·la·tive /-‘stra-p&-“lA-tiv/ adjective
– ex·trap·o·la·tor /-“lA-t&r/ noun