Definition - An infinitive when it is preceded by the particle to — for example, to be and to sit.
Note: When an infinitive is not preceded by the particle to (e.g. I love you), it is called a bare infinitive .

Oxford English Dictionary -
The term's first citation is from 1946:
"There is no reason to have a separate name for the to-infinitive."
(O. Jespersen Mod. Eng. Gram. V. 154)

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