A bundle of cross-linked stereocilia, arranged around a kinocilium on the apical surface of a sensory hair cell (e.g. a neuromast, auditory or vestibular hair cell). Stereocilium bundles act as mechanosensory organelles by responding to fluid motion or fluid pressure changes. [ http://www.ncbi.nlm.nih.gov/pubmed/7840137 http://www.ncbi.nlm.nih.gov/pubmed/15661519 ]