Word: Hick's Law
Definition: The time it takes for a person to make a decision increases with the number of choices available.
Reference: The law is named for British psychologist William Edmund Hick. Countless studies in fields from psychology to marketing have investigated the effect of options on decision making and satisfaction (I suggest the jam study if you're looking for a good example). Widespread consensus shows that not only do fewer choices decrease the time of decision making (a la Mr. Hick) but it also generates greater user satisfaction. The fewer the choices, the more satisfied the user is with her final decision. Seems counter intuitive, but science doesn't lie, my friend.
Thoughts: While it is simple to see how Hick's Law is used in web design to justify menu and navigation decisions, you would be limiting yourself greatly if that is the only design feature influenced by this principle. If you dig more deeply, you will see that decisions are the crux of experiences and impact every move and click a user takes.