The function terms is a generic function
  which can be used to extract terms objects
  from various kinds of R data objects.
terms(x, …)object used to select a method to dispatch.
further arguments passed to or from other methods.
An object of class c("terms", "formula") which contains the
  terms representation of a symbolic model.  See
  terms.object for its structure.
There are methods for classes "aovlist", and "terms"
  "formula" (see terms.formula):
  the default method just extracts the terms component of the
  object, or failing that a "terms" attribute (as used by
  model.frame).
There are print and labels methods for
  class "terms": the latter prints the term labels (see
  terms.object).
Chambers, J. M. and Hastie, T. J. (1992) Statistical models. Chapter 2 of Statistical Models in S eds J. M. Chambers and T. J. Hastie, Wadsworth & Brooks/Cole.