In BrE, 'dissertation' tends to be shorter (say 10,000 words), and 'thesis' tends to be longer (say 40,000-80,000 words). In AmE it is the opposite (a dissertation is longer, a thesis is shorter). Undergraduates can be said to write 'dissertations' (as tunaafi mentioned); you might also encounter 'final year dissertations' or 'honours ...
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In BrE, 'dissertation' tends to be shorter (say 10,000 words), and 'thesis' tends to be longer (say 40,000-80,000 words). In AmE it is the opposite (a dissertation is longer, a thesis is shorter). Undergraduates can be said to write 'dissertations' (as tunaafi mentioned); you might also encounter 'final year dissertations' or 'honours ...