What is a Seiko 5 Watch?

A great marketing idea from the 70's ~

To be a Seiko 5, the watch has to have the following five attributes:

1. Automatic Mechanism
2. Water Resistant (usually 30m)
3. Shock Resistant
4. Date Display
5. Day Display

Some Seiko 5's are in fact Seiko 4's, if the "Day" function is omitted