A disengaged (aka un-engaged) contact is one who has an engagement score of 0.
Engagement is calculated based on the actions of a contact who has been sent campaign(s). If a contact has an engagement score of 0, they are disengaged because they have not opened or clicked or forwarded your email ever. For more specific information on engagement score calculations, please visit this article.
Enterprise customers have the option to enable Advanced Engagement Scoring which scores based on demographic and subscriber action factors.
New contacts do not automatically have an engagement score until they've been sent a campaign.
For more strategic help, please contact your Client Success team member.