Hereinafter is even more official than herein. It means “from this point on, later in this same document.” Contracts love it because it lets them give a short name and then keep using it: “East Tea Company Limited (hereinafter ‘the Seller’).” After that line, every “the Seller” means that company. Though actually, it is not a word for “from now on” in daily life. That is from now on, later, or afterwards. Hereinafter only makes sense when there is a written text with a “here” (this document) and an “after” (the rest of the pages). It does not mean “after you die” or “in the afterlife,” and it does not mean “after this meeting” unless the meeting has produced a formal paper. You will see it in contracts, notices, and old-fashioned official English; you will almost never hear a friend say it.