"Sustained winds" refers to the average speed of the wind over a certain period of time, indicating how consistently strong the winds are blowing without consistent variation or interruption. Full definition
The forecast predicts between four and eight named storms may become hurricanes — organized, rotating storms with sustained winds of 74 mph (119 km / h) or faster. (scientificamerican.com)
The category is only based on maximum sustained winds. (climatesignals.org)
The average season sees 12 named storms, including six hurricanes, three major with sustained winds greater than 110 mph. (sun-sentinel.com)