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In my dictionary, `理系{りけい}`, `理科{りか}`, and `科学{かがく}` are defined simply as
"science".

What, if anything, differentiates them?

Especially in terms of the feeling or nuance when used in everyday
conversation.
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2012-02-03T04:43:46.810
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2012-02-04T09:07:18.360
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question
word-choice definitions nuances
What's the difference between 理系{りけい} , 理科{りか}, and 科学{かがく}?
Excuse me for posting an answer without referring to any sources...  
  
We usually use 理科 to refer to a school subject (which usually includes these 4
fields: physics, biology, earth science and chemistry). We usually use the
word 理科 in upper grades at elementary school, and in senior high we have
[理科第１分野]{りかだいいちぶんや} (=chemistry and physics) and [理科第２分野]{りかだいにぶんや} (=earth
science and biology). In senior high we usually choose 2 to 4 from these 4
fields and don't use the word 理科 anymore. I think we sometimes call these 4
subjects '[理科系科目]{りかけいかもく}...maybe it's like 'the group of scientific
subjects'/'one of the scientific subjects'.  
  
And as Paul-san said, we often say 'She is 理系/あの子は理系だよ。', 'I am
文系[ぶんけい]/私、文系なの。' when we talk about which field of 理系 and 文系 someone
majors/majored in at college, and also 'あそこの大学は文系だ/That university is 文系',
'ここの[学部]{がくぶ}・[学科]{かっか}は理系です/This faculty/department (at college) is 理系' etc.
to talk about what kind of courses a college/faculty/department has.  
  
As for 科学… sorry Dave-san, I still don't know what to write about it here...
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2012-02-04T08:57:09.153
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2012-02-04T09:07:18.360
2012-02-04T09:07:18.360
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answer