“The extent to which seed is free [or below a predefined threshold] from or carries pests and diseases. The term is sometimes used to refer to the germination rate (proportion of planted seed that begins to grow in a given period) and vigor (how well the plants grow) [in the case of grains mostly].” [see also seed degeneration].

References 

Sperling, Louise. 2008. When Disaster Strikes: A Guide to Assessing Seed System Security. CIAT Publication. Cali, Colombia: CIAT. http://ciat-library.ciat.cgiar.org/Articulos_CIAT/sssa_manual_ciat.pdf