entasis
noun
en·ta·sis ˈen-tə-sis 
plural entases ˈen-tə-ˌsēz
: a slight convexity especially in the shaft of a column
Word History
Etymology
Greek, literally, distension, stretching, from enteinen to stretch tight, from en- en- entry 2 + teinein to stretch — more at thin
First Known Use
1664, in the meaning defined above