It's called "a bare Infinitive".
According to English Grammar:
"When an infinitive is used without the marker to it is called a bare infinitive.
The infinitive is used without to after certain verbs like bid, let, make, see, hear, need, dare etc."
This bare infinitive is a part of "the Objective-with-the-Infinitive construction".
According to English Grammar:
"The Objective-with-the-Infinitive construction is a construction in which the Infinitive is in predicate relation to a noun in the Common Case or to a pronoun in the Objective case:
I saw him cross the street."