Full Definition of DOWNSTREAM. 1. : in the direction of or nearer to the mouth of a stream. 2. : in or toward the latter stages of a usually industrial process or the stages (as marketing) after manufacture.See downstream defined for English-language learners.See downstream defined for kids.ADVERTISEMENT.ull Definition of DOWNSTREAM. 1. : in the direction of or nearer to the mouth of a stream. 2. : in or toward the latter stages of a usually industrial process or the stages (as marketing) after manufacture. See downstream defined for English-language learners. See downstream defined for kids. ADVERTISEMENT.