Intro and Usher in

Intro noun - A short section (as of a book) that leads to or explains the main part.

Usher in and intro are semantically related in start topic. In some cases you can use "Usher in" instead a verb "Intro", when it comes to topics like initiate.

Usher in verb - Be a precursor of.

Intro and usher in are semantically related in start topic. Sometimes you can use "Intro" instead a verb "Usher in", if it concerns topics such as initiate.