"simple, uninflected form of a verb," 1510s (mid-15c. as an adjective), from Late Latin infinitivus "unlimited, indefinite," from Latin infinitus (see infinite). "Indefinite" because not having definite person or number.
The simple or dictionary form of a verb: walk, think, fly, exist. Often the word to marks a verb as an infinitive: “to walk,” “to think,” “to fly,” “to exist.”