FO (complexity)
FO is the complexity class of structures which can be recognised by formulae of first-order logic. It is the foundation of the field of descriptive complexity and is equal to the complexity class AC0 FO-regular. Various extensions of FO, formed by the addition of certain operators, give rise to other well-known complexity classes, allowing the complexity of some problems to be proven without having to go to the algorithmic level.
Read more about FO (complexity): Iterating, Logic Without Arithmetical Relations