Cibiyar mass kalma ce ta kimiyyar lissafi wacce ke nuni da batu a cikin wani abu ko tsarin da yawan abin ya taru kuma ana iya daukarsa a matsayin batu guda. Matsakaicin matsayi ne na dukkan sassan tsarin ko abu, wanda aka auna bisa ga adadinsu. Hakanan ana kiran tsakiyar taro a wani lokaci a matsayin cibiyar nauyi. A cikin sharuddan lissafi, ana iya ƙididdige tsakiyar taro ta amfani da matsakaicin ma'aunin nauyi na duk sassan tsarin ko abu.