Reductionism is the idea that knowledge at a higher level can be deduced from the entities and their interaction at a lower level
there are multiple meanings to reductionism, and this is one of the stronger ones. There are weaker forms of reductionism. The way I like to phrase it is usually "ontological reductionism" vs "explanatory reductionism". An ontological reductionist need not commit to the idea that all higher level patterns are understandable in terms of lower level stuff, even if it's ontologically true that each instance of them is caused by lower level stuff happening.