en·ta·sis 
(ĕntə-sĭs)
(ĕntə-sĭs)n. pl. en·ta·ses (-sēz′)
A slight convexity or swelling, as in the shaft of a column, conventionally employed especially in classical architecture.
[Latin, from Greek, tension, from enteinein, to stretch tight : en-, intensive pref.; see EN-2 + teinein, to stretch; see ten- in the Appendix of Indo-European roots.]
entasis