The infinitive is the base form of a verb. The terms base form and infinitive, or infinitive form are used interchangeably.

This is the form of the verb you would use to complete the sentence 'I want to...', e.g., to be, to go, to have, to leave, etc.

A full infinitive is with to. The bare infinitive is without to, as used after modal verbs.

Each verb actually has four infinitive forms:

Present infinitive: (to) go

Continuous infinitive: (to) be going

Perfect infinitive: (to) have gone

Perfect continuous infinitive: (to) have been going