Something impressionistic gives you a general feeling of a thing, not a careful list of facts. A report can be impressionistic: after a short visit, you say how the place felt, without numbers. A painting can be impressionistic too — loose colour, light, a mood — and that is where the word was born, from the art style Impressionism. In school and office English, though, people mostly use it for writing and thinking that is sketchy: “This is only an impressionistic account.” It is not an insult every time; sometimes a first look should be loose. But if a teacher asks for evidence, impressionistic is a polite warning: you felt it, you did not prove it. It is a fairly educated word. Though actually, everyday life runs on impressionistic judgments all the time — we size up a room in two seconds — we just do not call them that.