Thereinafter means coming later in the same thing. Thereinafter often refers to sections of text in a larger work.
The Supreme Court case of Loughborough v. Blake (1820) states “The 8th section of the 1st article gives to congress the ‘power to lay and collect taxes, duties, imposts and excises,’ for the purposes thereinafter mentioned" - referring to the rest of Article 1, Section 8 of the U.S. Constitution.
[Last updated in June of 2024 by the Wex Definitions Team]