When a journal requires that the work has not been published before except as a "preprint", is an arXiv publication considered a preprint? In the copyright transfer agreement of a journal there is the following sentence: 


  "The Author warrants that the Work has not been published before in
  any form except as a preprint."


Nowhere do they mention ArXiv specifically, but does "published to ArXiv" fall under "published as a preprint"?
Usually depositing a paper in arXiv prior to journal submission does count as a preprint. Wikipedia explicitly says so...here's the wiki link: https://en.wikipedia.org/wiki/ArXiv.

You can check out this list of journals with preprint policy: https://en.wikipedia.org/wiki/List_of_academic_journals_by_preprint_policy
If you posted your paper in arXiv before submitting to the journal, do inform the journal (include the link to your paper in arXiv). From your post, it seems that your journal does not have such a tight preprint policy.