It's the piece at the business end of the hose where the water comes out.

See these dictionary entries, for example:

http://books.google.es/books?id=ujs6bub-ai0C&pg=PA464&lpg=PA...

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Note added at 17 mins (2013-07-13 17:30:36 GMT)
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There are also some previous questions on this, now that I look:

http://www.proz.com/kudoz/spanish_to_english/engineering_gen...

http://www.proz.com/kudoz/spanish_to_english/energy_power_ge...

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Note added at 19 mins (2013-07-13 17:32:53 GMT)
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And then compare these images ("lanza"+"manguera"):
https://www.google.es/search?q="lanza" "manguera"&num=100&si...

with these ("nozzle" "hose"):
https://www.google.es/search?num=100&q="nozzle" "hose"&um=1&...