Step 1: In a normal diploid organism, every chromosome is present in exactly two copies, and this baseline condition is called disomy.
Step 2: Aneuploidy is specifically defined as any deviation away from this normal chromosome number for one or a few chromosomes, not a deviation for the whole genome as in polyploidy.
Step 3: Monosomy, trisomy and tetrasomy are all deviations, having one less, one more, or two more copies respectively, so all three qualify as aneuploid conditions.
Step 4: Disomy, being the unchanged normal state, is the odd one out, so the answer is \[\boxed{\text{disomy}}\]