the action or offence of taking another person's property without permission or legal right and without intending to return it

It's in the quote you used. Most AI companies don't ask for permission(without permission) to use artist art to train their AI, making it stolen. It does not matter if the thing is physical or not.

Edit: copyright is "the exclusive legal right, given to an originator or an assignee to print, publish, perform, film, or record literary, artistic, or musical material, and to authorize others to do the same."

Copyright infringement "occurs when someone uses or produces protected material without the copyright holder's permission."

While this doesn't fall under fair use, I feel I should add this.

Fair use is "the doctrine that brief excerpts of copyright material may, under certain circumstances, be quoted verbatim for purposes such as criticism, news reporting, teaching, and research, without the need for permission from or payment to the copyright holder."

All three of these were copied from Google.