In mathematical logic, a ground term of a formal system is a term that does not contain any variables. Similarly, a ground formula is a formula that does not contain any variables.

In first-order logic with identity, the sentence is a ground formula, with and being constant symbols. A ground expression is a ground term or ground formula.


Consider the following expressions in first order logic over a signature containing a constant symbol for the number a unary function symbol for the successor function and a binary function symbol for addition.

Formal definition

What follows is a formal definition for first-order languages. Let a first-order language be given, with the set of constant symbols, the set of (individual) variables, the set of functional operators, and the set of predicate symbols.

Ground terms

A ground term is a term that contains no variables. Ground terms may be defined by logical recursion (formula-recursion):

  1. Elements of are ground terms;
  2. If is an -ary function symbol and are ground terms, then is a ground term.
  3. Every ground term can be given by a finite application of the above two rules (there are no other ground terms; in particular, predicates cannot be ground terms).

Roughly speaking, the Herbrand universe is the set of all ground terms.

Ground atom

A ground predicate, ground atom or ground literal is an atomic formula all of whose argument terms are ground terms.

If is an -ary predicate symbol and are ground terms, then is a ground predicate or ground atom.

Roughly speaking, the Herbrand base is the set of all ground atoms, while a Herbrand interpretation assigns a truth value to each ground atom in the base.

Ground formula

A ground formula or ground clause is a formula without variables.

Formulas with free variables may be defined by syntactic recursion as follows:

  1. The free variables of an unground atom are all variables occurring in it.
  2. The free variables of are the same as those of The free variables of are those free variables of or free variables of
  3. The free variables of and are the free variables of except

See also