Japanese term (edited): 敷水、敷き水、しき水

It seems to mean the water put into a container (like a liquid prescription bottle) before the rest of the medicinal ingredients are added; this can prevent reactions between concentrated solutions, prevent ingredients from sticking to the bottom, and help speed up dissolving.

"しき水とは、濃厚溶液同士の相互作用による変化を防ぐため、あらかじめ薬瓶に全量の 1/5 程度入れる水のことである。"
http://www.daiichi-cps.ac.jp/images/material/2/files/jissyu2...

"ちなみに、ご存知と思いますがこうした際にあらかじめ測りとっておく水のことを「敷き水」と言います。"
http://intelligent-phram.seesaa.net/article/432674630.html
https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q12...
https://mixi.jp/view_bbs.pl?comm_id=51655&id=56469940

"The base liquid for the vast majority of mixtures is water. To this is added the medicinal
ingredients, substances to ensure even dispersion (if necessary), preservatives and flavourings. "
https://www.rpharms.com/Portals/0/MuseumLearningResources/03...