Any process that results in a change in state or activity of a cell or an organism (in terms of movement, secretion, enzyme production, gene expression, etc.) as a result of a serotonin stimulus. [ http://www.ncbi.nlm.nih.gov/pubmed/1505525 http://purl.obolibrary.org/obo/go/references/0000071 GOC:TermGenie ]