A weekday is a day of the week that is part of the workweek. Generally, with respect to labor in most parts of the world, a week is broken up into weekdays and the weekend, where weekdays are designated as days during which people work, and weekends are days for rest. Most typically, in a 7 day week, 5 days are weekdays.
Weekdays and workweeks
In the United States, the week is broken up as follows:
Weekdays: Monday, Tuesday, Wednesday, Thursday, Friday
Weekends: Saturday, Sunday
The division of the weekday and weekend above is quite common throughout the world, but there are also regions that follow a different schedule. In some countries, weekdays are Sunday through Thursday. Others may only have 4 weekdays, or up to 6 weekdays.
In most countries, including the US, working 8 hours a day, 5 days a week, for a total of 40 hours per week, is considered a "typical" full-time work schedule. When someone says they work a typical 9 to 5 job, this is what they mean.
A typical workweek varies between countries though, and within countries as well. Many people work longer than 40 hours a week, or have schedules that differ significantly from the what is considered the typical 9 to 5 weekday.
A 9/80 is one example of a different division of the workweek. In a 9/80, employees work 80 hours in 9 days over a period of 2 weeks. Usually this means getting every other Friday off. Others may work 10 hours a day, 4 days a week. There are many other work schedules in use, not all of which are centered around working 40 hours per week.