| Definition -
The infinitive when it is not preceded by the particle to — for example, be and sit.
1. When the infinitive is preceded by the particle to (e.g. to be and to sit), it is called either a full infinitive or a to-infinitive.
Etymology - The word infinitive derives from the Latin infinitivus, unlimited or indefinite.
Note: Diomedes called the Latin version the infinitivus, because it doesn't have a definite person or number.
Oxford English Dictionary - Its first citation is from 1520:
"Quis, qui, is … gouerned … somtyme of ye infinytyue mode folowynge."
(Whitinton Vulg. (1527) 3)