A post whose source is unavailable because the artist's account is suspended, deactivated or deleted. Not to be confused with Bad Id - although the two tags largely overlap, some platforms may keep posts accessible. Bad ID is about deleted sources while Inactive Account is about deleted accounts.
For Twitter, this can be detected if a post link shows the message: "This Post is from a suspended account".
For Tumblr, this can be seen in the blog's URL, as it will have -deactivated appended to the end of the URL, followed by the date of deactivation.