In mathematical logic, a first-order predicate is a predicate that takes only individual(s) constants or variables as argument(s).1 Compare second-order predicate and higher-order predicate.
This is not to be confused with a one-place predicate or monad, which is a predicate that takes only one argument. For example, the expression "is a planet" is a one-place predicate, while the expression "is father of" is a two-place predicate.
References
References
- Flew, Antony (1984), A Dictionary of Philosophy: Revised Second Edition, Macmillan, p. 147, ISBN 9780312209230.