MANDATORY - adjective - Required by law or mandate; compulsory. "Wearing helmets was made mandatory for pedal cyclists." (24th September)
 
POIGNANTadjective - Evoking a keen sense of sadness or regret. "A poignant reminder of the passing of time." (17th September)
 
SUSTAINABILITYnoun - The ability to be maintained at a certain rate or level. "The sustainability of economic growth." (10th September)
 
SPORADIC - adjective - Occurring at irregular intervals or only in a few places; scattered or isolated. (3rd September)

CCS Recommended Reading