sentence difference

(1) "Throughout this book the term
pressure refers to absolute pressure unless explicitly stated otherwise."
(2) "Throughout this book the term
pressure refers to absolute pressure unless explicitly stated."

If "otherwise" is omiited, it is still correct grammatically.

The meaning of the two sentences are the same, although there is a slight difference in the instruction.

Sentence 1 asks the reader, when encountering the word "pressure", to take 3 action:
(1) if the word is not further qualified, substitute it with the meaning of "absolute pressure", otherwise,go to step 2.
(2) if the word is further qualified, and if it is "absolute pressure", don't substitute, just leave it alone,
(3) if the word is further quaified, and if it is not "absolute pressure", leave it alone.


Sentence 2 asks the reader, when encountering the word "pressure", to take 1 action:
(1) if the word is not further qualified, substitute it with the meaning of "absolute pressure", otherwise, don't take any further action.

Therefore, sentence 1 may sound more clear, but actually adds unnecessary instruction steps, since it asks the reader to check the content of the "explicit statement" in steps 2 and 3, and then take no action. Sentence 2 is simplier, as it asks the reader to stop taking any further action after step 1.

However, I'd like to emphasize that the two sentences are both correct, and convey the same meaning.

Dear chunwailiu,
If" otherwise is omitted, the sentence is still gram
-matically correct, but missing something you want to tell the other
party. The different can be easily seen by comparing " He is a man"
and " He is a tall man "
Please note that "otherwise" can be used as an adverb,
an adjective or a conjuntion, and in this sentence, it apparently is
used as an adjective to add a meaning, namely, something different
from or opposite to, the thing ( absolute pressure) which has been
said; without it, the sentence is practically meaningless in the way
that you can't tell the other party exactly what you want to tell him
or her.

2009-06-21 19:30:07 補充：
I apologized for misspelling the word diferent for difference.