In my Spanish class, we learned that the verb for "to stop" is "parar", but many stop signs have the word "alto" on them instead of "pare" as the command form of "parar". I asked why, but no one was sure. Why is "alto" on a stop sign? What does it mean?
The word "alto" as an exclamation has nothing to do with "alto" as an adjective. In the XVI century, king Charles I of Spain also became emperor Charles V of the Holy Roman Empire, Lord of the Netherlands and Archduke of Austria. So in terms of Linguistics it is interesting to note how some words passed from a language in the Holy Roman Empire, such as German, to Spanish. One of them was the German word "halt", which became "alto", as noted by the first Spanish dictionaries from the RAE. The following is from 1770:
ALTO. (Milic.) La voz de que se usa para mandar parar la tropa ó suspender la accion en que está.
[Word used to command the troops to stop moving or to stop doing the current action.]
And before that, in 1726 it was:
ALTO. Voz que se usa para imponer siléncio, y que no se prosiga una conversación, discurso, riña, etc. aludiendo à la voz Alto en el uso militar.
[Word used to make someone shut up and not continue a conversation or speech, dispute, etc., alluding to the same word in its military usage.]
In a dictionary from 1607 it is registered the expression "alto hazer" as originated from the German word "Halten". You can still say "hacer un alto en el camino" as "to make a stop in your journey". And in 1591 the expression "alto hazer" was registered in a Spanish-English dictionary as "to keepe aloose".
So its use is indeed registered in the military context. Nowadays it still carries the same sense of obligation to stop, extended to non-military contexts.
Alto as a noun is the name of the stop sign in some parts of America; as an exclamation ("¡Alto!") it means "stop!" in all the Spanish speaking countries.
See Alto in the Diccionario de la Lengua Española (it comes from the German "Halt", from the German verb "halten").