If someone who has been doing something such as talking stops or pauses in midstream, they stop doing it, often before continuing. I was cut off in midstream. Midstream is also an adverb. The most difficult thing in a fast game of rugby is to change course midstream
Someone or something that is in midstream is in the middle of a river, where the current is strongest. Their boat had capsized in midstream. Midstream is also an adverb. Some of them got caught midstream by the tide