Cause neutrality (sometimes called cause impartiality[1] or strategic cause selection[2]) is the view that causes should be prioritized based on impartial assessments of impact rather than on other considerations, such as saliency or personal attachment. It is generally considered to be a core idea in effective altruism.
Suppose you value the welfare of all humans equally and suppose that dementia research is not the most effective way to help humans. Then being cause-impartial means you should not fund dementia research, even if dementia has personal significance to you (e.g. because it affected a family member).
Note that the implications of neutrality vary with the value system that one holds. If the person in the above example instead placed special weight on the welfare of those suffering from dementia, then supporting dementia research might actually be the best course of action.