The thesis is the (more or less) explicitly stated argument of the essay, while the dominant impression--which is often used to denote descriptive/illustrative essays--is the combined effect upon the reader, of all of the elements of the piece.
There is a stated difference between "dominant impression" and "thesis statement." While they are related, I can see where the distinction, and eventual confusion, would lie. In descriptive writing pieces, a dominant impression is the perception that you are trying to convey to the reader. The thesis statement is the one or two sentenced clear belief that communicates the sum total of your impressions to the reader. For example, if you are describing a visit to the hospital and you are striving to impart a feeling of gloom to the reader, the dominant impression would be all the details that contributed to this experience while the thesis statement would be your precise statement that clearly articulates your impression of your visit to the hospital. Dominant impressions are helpful because they create the atmosphere which will give birth to the thesis statement. The dominant impression is the garden and the surroundings that allow the flower of the thesis statement to bloom... or something like that.
All good writing must have a thesis, a central point that will be substantiated with the paper. A clear, specific and appropriate thesis statement renders a clear vision of the paper's purpose, into which all the dominant impressions will be channeled and filtered. The qualities of a thesis statement are that it is clearly worded, defined in a context that can be substantiated and supported with detail throughout the paper, and defines the boundaries of the work. Essentially, the thesis statement is the one sentence that permeates the entire paper; every single word, every phrase, every sentence resonates and is clearly related to that thesis statement. Clear language, strong focus, and words that make sense to both the reader and the writer help to make a great thesis statement, through which all the dominant impressions are channeled for meaning and purpose.