Taxonomy has two closely related meanings. First, it is the science of sorting living things into groups — kingdom, phylum, class, order, family, genus, species — and giving each one a name. That is the meaning you meet in biology class. Second, and just as common, a taxonomy is one particular system of groups that someone has built: 'a taxonomy of learning styles', 'the taxonomy used by this online shop'. In that second use it is countable, so you can say 'a taxonomy' and 'three taxonomies'. Outside biology the word has spread a long way. Teachers talk about Bloom's taxonomy of thinking skills; librarians, website designers, and business people use taxonomy to mean the set of categories and labels that make a big pile of information findable. It sounds formal — you would say it in a report or a meeting, not while chatting with friends. One honest note: a taxonomy is always somebody's decision about where to draw the lines, not a fact sitting in nature waiting to be picked up. Nature is messy, and the groups are our attempt to make sense of it.