She was under the impression that the meeting started at 3 p.m.

under 是介词，表示"在...之下"，这里用于固定搭配。 the 是定冠词，表示特指某个印象。 impression 是名词，意思是"印象、感觉"。 under the impression 是固定短语，表示"有...的印象/以为..."，后面常接that从句说明具体想法。