The definition from HubSpot is: A contact will be categorized as unengaged if they meet one of the following criteria: Never engaged with a marketing email from you and hasn’t engaged with the last 11 emails you’ve sent them. Previously engaged with one of your marketing emails but haven’t engaged with the last 16 emails you’ve sent them.

Thalox “Inactive or Indecisive Segments” predicts the likelihood someone is engaging with your next email.