IPA: /ˈmɪdstriːm/
KK: /mɪdˈstrim/
The central or middle section of a stream or process, often referring to a point that is not at the start or the end.
The fish are most active in the midstream of the river.
Midstream → It is formed from "mid-" (meaning middle) and "stream" (from Old English "stream", meaning a flow of water). The word "midstream" refers to the middle part of a stream or flow.
Think of being in the 'middle' ('mid-') of a 'flow of water' ('stream') — that's what midstream means.