First order predicate

In mathematical logic, a first-order predicate (also called a monad) is a predicate that takes only individual(s) constants or variables as argument(s). Compare second-order predicate and higher-order predicate.

A one-place predicate 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.