Midstream means in the middle of doing something, often at an awkward or difficult moment to stop. When you change your mind midstream while writing an essay, you might realize halfway through that your main argument doesn't work and need to start over. When someone switches topics midstream during a conversation, they suddenly start talking about something completely different before finishing their first thought.

If you're wading across a creek and decide midstream that the water's too cold, you're already committed: going back is just as hard as pushing forward.

You'll often hear phrases like “changing horses midstream,” which means switching your approach or plan when you're already partway through something. For example, if you're researching dolphins for a report and suddenly decide midstream to switch to sharks instead, you may have wasted all that work on dolphins. Sometimes changing direction midstream is necessary, but it usually means extra effort and complications that could have been avoided with better planning at the start.

As a noun, midstream can also mean the middle of a river or stream.