IPA: /hɪəˈraɪnˌætə/
KK: /hɪrˈaɪnˌædər/
In a later part of this document or text.
The terms outlined in this agreement will be defined hereinafter.
Hereinafter is formed from "here" (from Old English "hēr", meaning in this place) and "inafter" (from Middle English "inafter", meaning after this). The word refers to something that will be mentioned or referred to later in the same document or context.
Think of 'here' meaning this place and 'inafter' meaning after this — together, they help you remember that 'hereinafter' refers to something mentioned later in the same context.