Sine qua non \sin-ih-kwah-NON; -NOHN; sy-nih-kway-\, noun
An essential condition or element; an indispensable thing.
However we choose to define a classic, a sine qua non is that the material lend itself to reinterpretation in the light of changing circumstances.
– Matthew Gurewitsch, “A Country of Lesser Giants“, New York Times, April 4, 1999