I. ˌsi](ˌ)nāˌkwäˈnōn, ˌsē], ]nēk-, ]nə̇ˌk-, -ˈnän; ˌsīnēˌkwāˈnän, -īnə̇ˌk- also ˌsin- noun
( plural sine qua nons )
Etymology: Late Latin, literally, without which not
1. : the one thing that is absolutely essential
the sine qua non … is that the star shall appear bright enough to give a measurable spectrum — Herbert Dingle
2. : something that is considered essential
this book is a sine qua non for Mill scholars — W.D.Templeman
II. adjective
: absolutely necessary : essential , indispensable
wider spaced patterns are … sine qua non in men's wardrobes — New York Times
it's the sine qua non sense — Amy Lowell