On or Off.
Unambiguous Signals: Consider Paul Revere, waiting for news of the attack of the British troops. He is expecting to see a signal lantern in the tower of Old North Church telling him how the British are attacking:
1.32456 if by land, 1.71922 if by sea.
The signal shines out! ...and Paul Revere's famous ride is delayed for several hours as he tries to figure out just how bright that signal is.
Lack of ambiguity is a tremendous advantage. The signal that Paul Revere was actually waiting for that night in 1775 was:
One (lantern) if by land, and two (lanterns) if by sea.
...an easily interpreted signal. All Paul Revere had to do was count. Such signals are called discrete because they have a fixed number of definite states. Sometimes the word digital is used to mean the same thing.
Why can an abacus be regarded as a digital computer.